Sort the key bindings alphabetically

This patch contains no logic changes.  It only rearranges all of the
calls to bbcode/make-key-binding so that they appear in alphabetical
order, slightly improving readability.
Sort bbcode/font-lock-keywords alphabetically

Again this is to increase readability by making it easier to find any
codes we may have missed.  This patch does not change any mode logic,
only some whitespace and the order of a few lines.
Add support for the [email] tag

This patch is meant to help writing posts on the LÖVE forum
(http://love2d.org/forum) but it is highly likely that other systems
using BBCode also support this tag.
Alphabetically sort the tag list in the README for readability
Add support for the [attachment] tag

This is a tag often seen on the LÖVE forums where people use it to
upload their games.
Merge branch 'love-tags'

* love-tags:
  Add support for the [attachment] tag
  Alphabetically sort the tag list in the README for readability
  Add support for the [email] tag
  Sort bbcode/font-lock-keywords alphabetically
  Sort the key bindings alphabetically
Add mnemonic devices in the README to help remember key-bindings
Add the [gvideo] tag from http://bbcode.org/