Add Thomas Mango's alias generator plugin

At version 7c80d2
Fix cleaner.rb SystemStackError on aliases

Applied tsmango/jekyll_alias_generator#13, with thanks to
@joelmeyerhamme for the fix.
Experiment with aliases

Page rule at CloudFlare will redirect everything from cbea.ms to
chris.beams.io with paths intact. If the path is an alias, it will be
meta-refreshed to it's fully expanded path.

Example:

 http://cbea.ms/a/hello -> 302 -> http://chris.beams.io/a/hello
 http://chris.beams.io/a/hello -> meta-refresh -> http://chris.beams.io/writing/hello-world

It would of course be ideal to have only one 302 hop and avoid the
in-browser refresh, but this appears to be the simplest/best arrangement
possible using Jekyll.