Rename output files

    find sass-spec tests -name 'output.*.css' \
        | perl -ne '/\.lib\.css/&&next;s/\n//;print"mv $_";s/(\d)\.(\d)/$1_$2/;print" $_\n"'
Unify tests format
Compute engine stats
Fix endpoint bug, better error handling, stats on tests

* Since the endpoint was modified `3_2` instead of `3.2` for example,
  all SassMeister requests (except `lib`) failed, fucking up all tests
  results. Now fixed with a new engine mapping.
* Raise an error when SassMeister content type is not acceptable, log
  the unexpected response in output file, and display error in stderr.
* Stats are now computed on individual tests instead of features.
New tests output according to previous changes
Comppute support and stats according to previous changes
Untrack and ignore temporary files
Ignore individual support files too
Remove fix for old Git
Moved header and intro to partials to clean the index
Reworked the structure of _data/engines.yml
Added a statistics section
Fixed a broken test
Updated _data/support.yml
Merge branch 'develop'