Add tengo support to RemoteNickFormat (#793)

This commit add support for using the result of a tengo script in RemoteNickFormat using {TENGO}
Also adds a new toml table [tengo] with key RemoteNickFormat and value location of the script.
This also moves the TengoModifyMessage from [general] to Message in [tengo]

Documentation:

RemoteNickFormat allows you to specify the location of a tengo (https://github.com/d5/tengo/) script.
The script will have the following global variables:
to modify: result
to read: channel, bridge, gateway, protocol, nick

The result will be set in {TENGO} in the RemoteNickFormat key of every bridge where {TENGO} is specified
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/contrib/remotenickformat.tengo

[tengo]
RemoteNickFormat="remotenickformat.tengo"

bridge/config/config.go

@@ -166,6 +166,11 @@ type Gateway struct {
 	InOut  []Bridge
 }
 
+type Tengo struct {
+	Message          string
+	RemoteNickFormat string
+}
+
 type SameChannelGateway struct {
 	Name     string
 	Enable   bool
@@ -190,6 +195,7 @@ type BridgeValues struct {
 	WhatsApp           map[string]Protocol // TODO is this struct used? Search for "SlackLegacy" for example didn't return any results
 	Zulip              map[string]Protocol
 	General            Protocol
+	Tengo              Tengo
 	Gateway            []Gateway
 	SameChannelGateway []SameChannelGateway
 }