Indented syntax: allow indentation to vary within a file

[Natalie Weizenbaum]

The Sass indented syntax is currently very strict about what indentation can be used. Once a single indented line has been parsed, all future lines must use multiples of that number of spaces. We should loosen this up, and only require that indentation be relatively higher or lower.

Note that we'll still require that a given document only use tabs or spaces, not both.