Add a new boolean parameter 'utf8' to VSEval

When this option is set (default: false) the source string is assumed to be UTF-8 encoded, instead of the system code page. That way, it's possible to write non-"ANSI" VapourSynth scripts on an AviSynth script.

Obviously it's not intended that users paste encoded text on their avs scripts. This option is useful for AvsPmod, for opening vpy scripts as if they were avs and silently evaluate them with VSEval. And apparently it's also useful for previewing on VirtualDub.