Permit customizable addition of rcirc-styles-map to rcirc-mode-map

[aaron-em]

Once the new features in #4 and #5 ship, there'll be a need for some straightforward way of adding them to rcirc-mode map which doesn't require manually doing so via Lisp.

Right now it looks like this would be best implemented as:

• a defcustom allowing the keyseq to be specified;
• a set function on same which will:
  • if replacing a binding which was already bound before we got to it, restore the prior binding
  • if a binding already exists in rcirc-mode-map on the given keyseq, capture it for later restoration as above
  • finally, bind rcirc-styles-map to the new key sequence in rcirc-mode-map

This way, we don't stomp all over rcirc-mode-map every time the user changes the prefix. We could even, in theory, actually hang the prior binding off a keyseq in rcirc-styles-map, so that it's still accessible via key sequence even after we've done our thing.

[benma]

Why is this issue an issue? I won't stop you from implementing it, but I am not sure how many users will benefit from being able to customize the prefix via Emacs' customization interface.

I think doing it the lisp way is fine for now. Up to you, of course.

[aaron-em]

I don't want to just unilaterally pick something and bind to it because I don't own rcirc-mode-map, and (potentially) blowing away one of its pre-existing bindings would be rude. I don't suppose there's any overwhelming reason why it needs to have a customization instead of just having the user drop a define-key form into her init file, but why not make the option available? It's not as though a defcustom is hard to write.

On the other hand, this isn't exactly related to what I'm doing in #4 and #5, and I want to go ahead and get that functionality into master, so I broke this out into a separate ticket for later attention.