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_process.go

@@ -0,0 +1,60 @@
+package stdlib
+
+import (
+	"os"
+	"syscall"
+
+	"github.com/d5/tengo/objects"
+)
+
+func makeOSProcessState(state *os.ProcessState) *objects.ImmutableMap {
+	return &objects.ImmutableMap{
+		Value: map[string]objects.Object{
+			"exited":  &objects.UserFunction{Name: "exited", Value: FuncARB(state.Exited)},   //
+			"pid":     &objects.UserFunction{Name: "pid", Value: FuncARI(state.Pid)},         //
+			"string":  &objects.UserFunction{Name: "string", Value: FuncARS(state.String)},   //
+			"success": &objects.UserFunction{Name: "success", Value: FuncARB(state.Success)}, //
+		},
+	}
+}
+
+func makeOSProcess(proc *os.Process) *objects.ImmutableMap {
+	return &objects.ImmutableMap{
+		Value: map[string]objects.Object{
+			"kill":    &objects.UserFunction{Name: "kill", Value: FuncARE(proc.Kill)},       //
+			"release": &objects.UserFunction{Name: "release", Value: FuncARE(proc.Release)}, //
+			"signal": &objects.UserFunction{
+				Value: func(args ...objects.Object) (ret objects.Object, err error) {
+					if len(args) != 1 {
+						return nil, objects.ErrWrongNumArguments
+					}
+
+					i1, ok := objects.ToInt64(args[0])
+					if !ok {
+						return nil, objects.ErrInvalidArgumentType{
+							Name:     "first",
+							Expected: "int(compatible)",
+							Found:    args[0].TypeName(),
+						}
+					}
+
+					return wrapError(proc.Signal(syscall.Signal(i1))), nil
+				},
+			},
+			"wait": &objects.UserFunction{
+				Value: func(args ...objects.Object) (ret objects.Object, err error) {
+					if len(args) != 0 {
+						return nil, objects.ErrWrongNumArguments
+					}
+
+					state, err := proc.Wait()
+					if err != nil {
+						return wrapError(err), nil
+					}
+
+					return makeOSProcessState(state), nil
+				},
+			},
+		},
+	}
+}