Add cache_impl and other cache settings to configuration

[danjac]

This would be useful especially as Beaker (the default) is being deprecated in favor of dogpile:

http://dogpilecache.readthedocs.org/en/latest/api.html#mako-integration

For example:

mako.cache_impl = dogpile.cache
mako.cache_regions = 
       default=myapp.mycache