Improve and simplify the CommaProxyList algorithm.

[duncf]

With this change CommaProxyList better handles and preserves non-standard (and even inconsistent) indentation, rather than assuming indented lists are indented by four spaces (fixes #100). It also is significantly faster in very large files by avoiding use of ".indentation", which can require iterating over every element in the file. End-of-list comments are preserved at the end of the list, and inline comments are also preserved.

[janlugt]

I have the same issue, could this please be merged?

[janlugt]

@Psycojoker, could you take a look at this?

[janlugt]

@Psycojoker, could you reconsider merging this and https://github.com/PyCQA/redbaron/pull/145? It dramatically improves the performance of the CommaProxyList algorithm.

[hectorv]

+1 The CommaList exception got me here.