Fix completing strings with a dash symbol

When string has a dash symbol ycm.CompletionStartColumn() method returned a index of a dash. It acts as a trigger, so completer thought that he must get a query after dash.

This actually affects not only filename completer, but all completers. 

This patch fixes this problem. However, one issue still left. Consider following string: ``~/.vim/bundle/-^``, where ^ is a cursor position. This will not return any completions, but if we add anything after dash it will work fine.

Partially fixes #281