Fix crash when typing in multibyte characters

When typing in multibyte characters like 'ö' and 'ä', LeaderF spits out error messages. The reason is that for the character 'ö' it creates a regexp like this: `[\xc3\xb6]` which matches every string that contains one of these two bytes (instead of strings that contain *exactly* these two bytes). That leads to false positives which produce these errors. My patch fixes it (and is a little bit easier ;)

By the way, LeaderF looks really promising so far. But a feature that I would really like to have, and all those CtrlPs and other CtrlP-like plugins lack, is a smooth way to browse through your file system. That means, without all the time including recursively all files in the current directory. Do you plan to include such a feature or should I try myself to add it to LeaderF?