Add scripting (tengo) support for every incoming message (#731)

TengoModifyMessage allows you to specify the location of a tengo (https://github.com/d5/tengo/) script.
This script will receive every incoming message and can be used to modify the Username and the Text of that message.
The script will have the following global variables:
to modify: msgUsername and msgText
to read: msgChannel and msgAccount

The script is reloaded on every message, so you can modify the script on the fly.

Example script can be found in https://github.com/42wim/matterbridge/tree/master/gateway/bench.tengo
and https://github.com/42wim/matterbridge/tree/master/contrib/example.tengo

The example below will check if the text contains blah and if so, it'll replace the text and the username of that message.
text := import("text")
if text.re_match("blah",msgText) {
    msgText="replaced by this"
    msgUsername="fakeuser"
}

More information about tengo on: https://github.com/d5/tengo/blob/master/docs/tutorial.md and
https://github.com/d5/tengo/blob/master/docs/stdlib.md

vendor/github.com/d5/tengo/stdlib/os_exec.go

@@ -0,0 +1,109 @@
+package stdlib
+
+import (
+	"os/exec"
+
+	"github.com/d5/tengo/objects"
+)
+
+func makeOSExecCommand(cmd *exec.Cmd) *objects.ImmutableMap {
+	return &objects.ImmutableMap{
+		Value: map[string]objects.Object{
+			// combined_output() => bytes/error
+			"combined_output": &objects.UserFunction{Name: "combined_output", Value: FuncARYE(cmd.CombinedOutput)}, //
+			// output() => bytes/error
+			"output": &objects.UserFunction{Name: "output", Value: FuncARYE(cmd.Output)}, //
+			// run() => error
+			"run": &objects.UserFunction{Name: "run", Value: FuncARE(cmd.Run)}, //
+			// start() => error
+			"start": &objects.UserFunction{Name: "start", Value: FuncARE(cmd.Start)}, //
+			// wait() => error
+			"wait": &objects.UserFunction{Name: "wait", Value: FuncARE(cmd.Wait)}, //
+			// set_path(path string)
+			"set_path": &objects.UserFunction{
+				Value: func(args ...objects.Object) (ret objects.Object, err error) {
+					if len(args) != 1 {
+						return nil, objects.ErrWrongNumArguments
+					}
+
+					s1, ok := objects.ToString(args[0])
+					if !ok {
+						return nil, objects.ErrInvalidArgumentType{
+							Name:     "first",
+							Expected: "string(compatible)",
+							Found:    args[0].TypeName(),
+						}
+					}
+
+					cmd.Path = s1
+
+					return objects.UndefinedValue, nil
+				},
+			},
+			// set_dir(dir string)
+			"set_dir": &objects.UserFunction{
+				Value: func(args ...objects.Object) (ret objects.Object, err error) {
+					if len(args) != 1 {
+						return nil, objects.ErrWrongNumArguments
+					}
+
+					s1, ok := objects.ToString(args[0])
+					if !ok {
+						return nil, objects.ErrInvalidArgumentType{
+							Name:     "first",
+							Expected: "string(compatible)",
+							Found:    args[0].TypeName(),
+						}
+					}
+
+					cmd.Dir = s1
+
+					return objects.UndefinedValue, nil
+				},
+			},
+			// set_env(env array(string))
+			"set_env": &objects.UserFunction{
+				Value: func(args ...objects.Object) (objects.Object, error) {
+					if len(args) != 1 {
+						return nil, objects.ErrWrongNumArguments
+					}
+
+					var env []string
+					var err error
+					switch arg0 := args[0].(type) {
+					case *objects.Array:
+						env, err = stringArray(arg0.Value, "first")
+						if err != nil {
+							return nil, err
+						}
+					case *objects.ImmutableArray:
+						env, err = stringArray(arg0.Value, "first")
+						if err != nil {
+							return nil, err
+						}
+					default:
+						return nil, objects.ErrInvalidArgumentType{
+							Name:     "first",
+							Expected: "array",
+							Found:    arg0.TypeName(),
+						}
+					}
+
+					cmd.Env = env
+
+					return objects.UndefinedValue, nil
+				},
+			},
+			// process() => imap(process)
+			"process": &objects.UserFunction{
+				Value: func(args ...objects.Object) (ret objects.Object, err error) {
+					if len(args) != 0 {
+						return nil, objects.ErrWrongNumArguments
+					}
+
+					return makeOSProcess(cmd.Process), nil
+				},
+			},
+		},
+	}
+}