Correctly split @media blocks across files
Be more flexible about where we can find the split-point

Only looking in the extname doesn't work so well if we're not the last
processor in the chain. Plus, if we extract it from the '_splitN' part,
we don't need to define separate extension processors for each.

Also, default to the second part: the zeroth part is not very useful,
and you can just use the un-split file if you want the first.
Merge branch 'media-queries' into future

* media-queries:
  Correctly split @media blocks across files
Merge branch 'alternate-filenames' into future

* alternate-filenames:
  Be more flexible about where we can find the split-point
Fix for closing media tags exactly on the split

This fixes the issue where the assets cannot be compiled because the end
of a media tag would be inserted exactly at the next new file.

In other words, it fixes this issue:
Invalid CSS after "@charset "UTF-8"; ": expected selector or at-rule, was "}"
Merge pull request #1 from VioletGrey/fix-closing_of_media

Fix closing of media