Enable 'beet mod' style tag definition with 'beet import'

[khimaros]

It would be useful to be able to tag all music while importing a new directory.

The behavior would be similar to the 'beet mod' command but applied during import.

For example:

% beet import /path/to/music special=mashups % beet import /path/to/other special=remixes

With some workflow, you can accomplish this now:

% beet import /path/to/music % beet mod -y '^special::\w+' special=mashups % beet import /path/to/other % beet mod -y '^special::\w+' special=remixes

However, this becomes unruly unless you use the same field for all imports.

[sampsyo]

Please see #1881.