Support for g:fzf_action to change hotkeys.

[noyez]

As a warning, i have never written vim files before so this is a first-cut.

With fzf, if you specify the following (which is the default):

let g:fzf_action = {
  \ 'ctrl-t': 'tab split',
  \ 'ctrl-x': 'split',
  \ 'ctrl-v': 'vsplit' }

It is possible to override the default hotkey for opening files from the fzf view using g:fzf_action.
In my case i have ctl-x already mapped to an action, so i cannot open
a file in split-view using fzf/skim. Changing the g:fzf_action allows
me to change the hotkey for split-view to ctrl-e (or anything)

let g:fzf_action = {
  \ 'ctrl-t': 'tab split',
  \ 'ctrl-e': 'split',
  \ 'ctrl-v': 'vsplit' }