Add instructions for Vim Ex command setup

[VadimBrodsky]

I didn't want to install the Vim plugin, but I want to run elm-format from Vim. This is a small Ex mode command that will run elm-format on a buffer save of an .elm file.

The first example is more verbose and adds an explicit :ElmFormat ex command as well as a leader key binding.

The second example just runs elm-format on save without defining an explicit Ex command.

[avh4]

Thanks! I'm hesitant to merge this, as I want to try to keep the install options as simple as possible for folks who are not experts with their particular editor. But I'll leave this issue open for reference by anyone who might be looking for this.