Add --always flag for write subcommand

Forces a write of tags to file even if the file's tags match the database.
This is useful to force plugins that respond to write (e.g., Scrub and Zero) to run on those tags.
This may also make the TODO comment in zero.py less important since creates a way to manually run the zero plugin on a file imported as-is.
Clarify the default behavior for the write subcommand
Change flag from --always to --force for symmetry with other beets commands
Merge pull request #906 from hkhanna/write-always-flag

Add --force flag for write subcommand