(GH125) Escape prefix character when evaluating messages

Any single character can be defined as the prefix character (default is "!") that indicates to PoshBot that the incoming message is a bot command. Some of these characters are also regex special characters and we need to escape them properly so when people indicate they want to use the '.' or '^' characters for example, they are treated a literal.

PoshBot/Classes/Bot.ps1

@@ -291,6 +291,7 @@ class Bot : BaseLogger {
      [bool]IsBotCommand([Message]$Message) {
         $parsedCommand = [CommandParser]::Parse($Message)
         foreach ($prefix in $this._PossibleCommandPrefixes ) {
+            $prefix = [regex]::Escape($prefix)
             if ($parsedCommand.command -match "^$prefix") {
                 $this.LogDebug('Message is a bot command')
                 return $true
@@ -519,7 +520,8 @@ class Bot : BaseLogger {
             $firstWord = ($Message.Text -split ' ')[0]
 
             foreach ($prefix in $this._PossibleCommandPrefixes) {
-                if ($firstWord -match "^$prefix") {
+                $prefixEscaped = [regex]::Escape($prefix)
+                if ($firstWord -match "^$prefixEscaped") {
                     $Message.Text = $Message.Text.TrimStart($prefix).Trim()
                 }
             }