fromfilename: Configurable extraction patterns

# Recommendations for manual editing imported tracks

A) the use of the plugin `fromfilename` (although I can't get it to work)
B) simple option to just edit

Option A
Upon importing a track (with option to skip matching from sources, because there isn't any), this plugin will provide you the option to set the format-pattern to define which goes where.

Because fromfilename plugin doesn't have any 'configuration' option like other plugins have, there isn't a way to verify the format used within the filename to embed the tags from.
But if you look at the beetsplug/fromfilename.py it has sections for field
extraction patterns for which could possibly define the format-pattern in this following scenario:

File (w/out any tags):
01. Mack 187 & Akoza - Introduction.flac

Format:
$track. $artist - $title

Option B
same as above but by not defining the fromfilename this instead will allow you to edit the general tags yourself:
-artist
-title
-album
-track/total
-disc/total
-comment
...etc

as also suggested from #154 @dokterbob recommended an similar 'Apply and edit' option, except mine includes what could be edited and that it should be up to the user to define what they want to have the option to edit from.