Support indented syntax for string input

[TamsynUlthara]

libsass supports the indented syntax, e.g.,

p
  a
    color: blue

This binding correctly handles files written in the indented syntax if passed as filenames with the .sass extension, e.g.,

sass.compile(filename="foobar.sass")

... but has no support for the indented syntax as string input, e.g.,

sass.compile(string="p\n  a\n    color: blue\n")

Perhaps a keyword parameter, e.g., syntax="sass" / syntax="scss"?

[asottile]

The setting in libsass's context is is_indented_syntax_src if you're interested in taking a shot at it :)