Allow reloading of the plugin (use bang with :fun/:com)

Original pull request:

  https://github.com/wincent/command-t/pull/139

To reload, you'll have to do something like:

  :unlet g:command_t_loaded
  :unlet g:command_t_autoloaded
  :source path/to/plugin/command-t.vim
  :source path/to/autoload/commandt.vim

Signed-off-by: Greg Hurrell <greg@hurrell.net>
doc: Update HISTORY for 'blueyed/allow-reload' change
Merge branch 'blueyed/allow-reload' into next

* blueyed/allow-reload:
  doc: Update HISTORY for 'blueyed/allow-reload' change
  Allow reloading of the plugin (use bang with :fun/:com)