Add fuzzy match flag

This is related to #88

I will have to check that following commands works well
- [x] helm-gtags-select
- [x] helm-gtags-select-path
- [x] helm-gtags-tags-in-this-function
- [x] helm-gtags-find-tag
- [x] helm-gtags-find-tag-other-window
- [x] helm-gtags-find-rtag
- [x] helm-gtags-find-symbol
- [x] helm-gtags-find-pattern
- [x] helm-gtags-find-files
- [x] helm-gtags-find-tag-from-here
- [x] helm-gtags-dwim
- [x] helm-gtags-parse-file
- [x] helm-gtags-show-stack
- [x] helm-gtags-resume