Add MapHelp command

[idbrii]

MapHelp makes it easier to lookup what a command does. Most of the time,
it returns whatever you've mapped the input key sequence to, but if it's
built-in it shows vimdoc for the map (instead of an error like :map).

It optionally uses Verbose and seems like a relevant addition to scriptease,
so I thought I'd propose and see what you thought.

Useful whenever you know you use something a lot, but can't remember
where it comes from.

Also includes a bang variant to use :Verbose instead of verbose.

Passing no modes will search all modes unlike :map or maparg(). This is
the most useful use case. You can pass a mode to pinpoint entries in
vimdoc or for Verbose.

[tpope]

I like this idea. My first instinct is to double down on the :help aspect and invoke :help on the underlying map (presumably a <Plug> map) rather than dumping raw :verbose output.

[tpope]

I should add that for my own plugins, I am terrible about documenting <Plug> maps, but this is a strong incentive to change that. In the case that no topic is found, we could include the relevant filename in an expanded error message: E159: Sorry no help for <Plug>whatever (/path/to/plugin/whatever.vim).