Upgrade to libsass==3.1.0. Resolves #36. Resolves #38.

[asottile]

Things this does:

• Upgrade to libsass==3.1.0 (Update to libsass 3.1.0 #38)
• Use the newer not-deprecated api
• Reimplement compile_dirname in python (it was dropped from the not-deprecated api)
• The reimplemented compile_dirname function replaces .scss with .css in the output directory (Add support for extension substitution #36)

This is a prerequisite to implementing #13

[asottile]

oops, forgot to run the py3 tests, one sec lemme fixup my test

[coveralls]

Coverage increased (+1.96%) to 72.04% when pulling 0259a57 on asottile:upgrade_to_cpp_api into 0cb44c3 on dahlia:python.

[coveralls]

Coverage increased (+1.96%) to 72.04% when pulling 0259a57 on asottile:upgrade_to_cpp_api into 0cb44c3 on dahlia:python.

[coveralls]

Coverage increased (+2.91%) to 72.99% when pulling 15df8fc on asottile:upgrade_to_cpp_api into 0cb44c3 on dahlia:python.

[dahlia]

Now does it become to have to write @charset "UTF-8"; in order to use non-ASCII characters?

[dahlia]

Anyway, huge thanks for your effort!

[asottile]

This seems to be the PR which fixes the bug of not having @charset: sass/libsass#717

[dahlia]

Okay, I will consider whether cherry-pick sass/libsass#717 or not, after merging this PR.

[asottile]

Here's my thoughts (I think you should keep it):

1. Currently we're pointing at vanilla 3.1.0. Picking patches out of that would mean we don't support the same feature set as 3.1.0 (and we'd stop being a simple wrapper).
2. Precedence:
   • npm node-sass: has @charset for files containing non-ASCII bytes
   • ruby-gem sass: has @charset for files containing non-ASCII bytes
3. This spec-set: https://github.com/sass/sass-spec/blob/master/spec/basic/45_str_insert/input.scss https://github.com/sass/sass-spec/blob/master/spec/basic/45_str_insert/expected_output.css