Added "ignore_whitespace" option

Allows the user to chose whether or not whitespace is ignored by GitGutter. Three options:
- "none" (default) retains the current default behaviour of including whitespace
- "all" ignores all whitespace, adds -w arg
- "sol" only ignores whitespace at the end of lines, adds --ignore-space-at-eol arg
Merge branch 'master' of github.com:mwillmott/GitGutter into mwillmot
Fix syntax error when adding ignore_whitespace arg
Merge branch 'master' of github.com:jisaacks/GitGutter