add support for different ctags arguments based on filetype

[huhumt]

Add support to use different ctags arguments based on filetype, I've tested on my .vimrc with this settings:

let g:gutentags_ctags_extra_args_dict = {
            \ 'c': [
                \ '--fields=+ailmnS',
                \ '--recurse',
                \ '--languages=C,C++',
                \ '--c++-kinds=+p',
                \ '--extras=+q'
                \ ],
            \ 'cpp': [
                \ '--fields=+ailmnS',
                \ '--recurse',
                \ '--languages=C,C++',
                \ '--c++-kinds=+p',
                \ '--extras=+q'
                \ ],
            \ }

With this modification, the vim plugin code parameter autocomplete will take effects.