Suggested api changes

[asottile]

I think it would simplify the code to expose the following instead of compile:

def compile_string(s, **kwargs):
    # ...

# and

def compile_file(filename, **kwargs):
    # ...

and drop compile_directory (libsass has hard deprecated it)

[dahlia]

Seems better, we can change it in 0.8.0 release.

[mgreter]

Libsass never implemented compile_directory and traces in the API will probably be removed completely. There are some internal refactorings pending which will remove this from the internal C++ API too. Was there anything related to this exposed on the C level? I was under the impression we could safely remove it from libsass.

[asottile]

@mgreter it used to be exposed in the old api, which I removed dependence on here: 202d657#diff-454523db191eb8e24222f01b7d4e7c1dL144

To keep our api compatibility, we have an implementation in pure python: https://github.com/dahlia/libsass-python/blob/599f47019da132008fecb168d272fd814ac85821/sass.py#L198