remove assertion fail caused by previous commit
docs: note about possible lagging packaged version

It's surprising how many Debian users don't realize they're looking at the
wrong version of the docs.
new file format? file a bug

(as opposed to emailing me directly)
docs: flexattrs guide in "advanced" guide
changelog reorganization
Merge pull request #448 from pprkut/write

#328 Add beet write command
slightly simpler "write" command (#328, #448)
docs/changelog for write (#328, #448)
changelog/thanks for 887e02c fix
opportunistic cleanup
lyrics -f (#455, closes #414): style, changelog
clutter option: permit whitespace-separated
replace: tolerate None replacements

Avoids confusion/traceback when you leave off the replacement string instead
of using ''.
mpdstats: clean up some unused code
mpdstats: style fixes
simplify MPD configuration
mpdstats: log with displayable_path, etc.
docs proofreading
duplicates: some config handling simplification
duplicates: only one type of deletion