Match acronyms greedily in the inflector.

Currently acronyms are matched in the order they are defined, which can
be a problem if you define acronyms that contain other acronyms. If you
for example define HTM as an acronym and then define HTML as an acronym
too, 'HTML'.underscore will return 'htm_l' which to me is confusing.

Furthermore, this will depend entirely on the order in which the
acronyms were defined - the inflector behaves as expected if HTML is
defined as an acronym before HTM.

This commit makes the acronym regex be sorted by length with the
longest acronym first so that the longest suitable acronym will be
matched regardless of the order in which the acronyms were defined.