Add more TODOs to fastdupes.py

- a --prefer option for automating duplicate deletion
- Look into the performance effects of using the multiprocessing module
- Look into using things like /proc/sys/vm/drop_caches to cold-cache test
Rename 'uniques' -> 'keep_uniques' in fastdupes.groupBy(...) args.

(It's more intuitive that way)
Rename *Grouper functions in fastdupes to *Classifier to be more accurate.
Add a Vim modeline to fastdupes.py
Fix a bunch of flake8 complaints.
Rework groupBy classifiers to not rely on side-effects.
Whoops. fastdupes was way too slow because it wasn't hashing heads.

(I accidentally passed limit in as want_hex and when calling hashFile()
 and, since Python's bool() is a subclass of int() for historical reasons,
 it didn't complain.)
Merge subgroupByContents into groupBy in fastdupes.py
Stop using hacky dynamic string formatting.

(Use C-style %*s dynamic padding instead)
Add setup.cfg to shut PyFlakes up about things I'm not going to change.
Re-run epydoc on fastdupes.py and fix a bunch of docstring mistakes.