add variable 'g:fakeclip_always_provide_clipboard_mappings'

Please provide a way for a user to fakeclip's clipboard mappings
even if `has('clipboard') == 1` .

It is useful for a user who disables clipboard access
like the followings:

    " When running on terminal, use |clipboard-exclude|
    " to disable connecting to X server
    " not to be killed with X server.
    if !has('gui_running')
        set clipboard+=exclude:.*
    endif