Simplify api

see usage:
* https://github.com/yongkangchen/atom-ctags/blob/simplify-api/lib/ctags-complete.coffee
* https://github.com/yongkangchen/atom-ctags/blob/simplify-api/lib/ctags-provider.coffee

diff: https://github.com/yongkangchen/atom-ctags/commit/33a60f4189984a016d5beef037c3f864ab98efaa