Allow functions to fetch nickserv passwords

* circe.el (circe-network-options): Update docstring.
(circe-nickserv-password): Update docstring.
(circe-nickserv-identify): Allow a function for the nickserv password.
Merge pull request #90 from capitaomorte/functions-for-nickserv-passwords

Allow functions to fetch nickserv passwords
Make tracking links in the mode line clickable.

A user will now be able to click on buffer names in the tracking
mode line when there is activity in them.

Thanks to João Távora for the initial pull request.
Implement italic control code
Merge pull request #92 from wasamasa/feature-italic-irc-control-code

Implement italic control code