profile management

[liushapku]

These commits introduce a new setup method which can handle more than one github accounts hosted by different github api url. The new method will have no effect for existing users unless they explicitly set some global variables (listed below, see :help gist-vim-setup-profile for details)

let g:gist_profiles = {
    \ 'github'     : ['', 'username'],
    \ 'enterprise' : ['https://mycompany.com/api/v3', 'employeeid']
}

let g:gist_token_dir = '~/.config/gist-vim/'   (default value)

let g:gist_default_profile = 'enterprise'      (no default value)

Two equivalent commands are introduced to switch between profiles. The second one has completefunc defined

  :Gist --profile [profilename]
  :GistProfile [profilename]

Any comment is appreciated.