Added -file (-f) option to start.ps1

This allows specifying a PocketMine-MP PHP file to execute (phar or source) to use to run the server, like start.sh. If not specified, auto-detection is used.

start.ps1

@@ -2,6 +2,7 @@
 param (
 	[string]$php = "",
 	[switch]$Loop = $false,
+	[string]$file = "",
 	[string][Parameter(ValueFromRemainingArguments)]$extraPocketMineArgs
 )
 
@@ -14,6 +15,7 @@ if($php -ne ""){
 	$binary = "php"
 }
 
+if($file -eq ""){
 	if(Test-Path "PocketMine-MP.phar"){
 	    $file = "PocketMine-MP.phar"
 	}elseif(Test-Path "src\pocketmine\PocketMine.php"){
@@ -23,6 +25,7 @@ if(Test-Path "PocketMine-MP.phar"){
 	    pause
 	    exit 1
 	}
+}
 
 function StartServer{
 	$command = "powershell " + $binary + " " + $file + " " + $extraPocketMineArgs