Custom implementation of import sorting.

[Carreau]

This tries to implement some of the rule of import sorting as requested in #13 using custom logic, This was originally fixed in #263 using isort, but reverted and re-requested as of #287

Current this has the following limitations/bugs that will need to be sorted out, but might need fixes to Pyflyby master branch first.

This can be tried with the --experimental-sort-imports

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• [ ] A) I believe we want from __future__ import ..., to always be followed by a blank line, it is currently not the case if the next lexicographic import is a single import.
• [ ] B) from __future__ import ... is not always inserted before the first blank lines in the file,
• [x] C) blank lines are often inserted before comments
• [ ] D) It also looks like the inserted from __future__ import... do not get the same treatment with respect to aligning with space, but it's unclear wether this is a bug (and it can be solved separately).

C, (and likely B), is I think a bug in Pyflyby parsing where new lines are part of the next statement (especially if it is a comment) instead of being part of the previous one – or being their own statement. It might affect strings literal as well (to check). See #328

From what I can tell this may change API, so I want to be careful fixing this.

[Carreau]

I managed to fix the whitespace issue which took a lot of refactor. Way too much, and way more than I expected.

As this is behind an experiemental flag I'm going to merge it, and we can iterate.