Added editor subcommand.

[br1tney5pear5]

This PR adds editor subcommand, subcommad is just another proc triggered by keybinding which then parses user input and executes the procedure user specified, e.g. :open or :filter. All named procedures are accessible from subcommand.

Furthermore args can be provided on invocation like :open data/fixed.csv. Open and filter procedures were adapted to only prompt user for input if the argument wasn't provided.

Lexer for the subcommand is smart enough to recognise strings with spaces and escapes as single tokes so e.g. this is completely fine :open "data/insane \" : file \" name.csv"

[liquidaty]

@br1tney5pear5 sounds great. Could you please add some tests that cover this new feature, that run as part of the test-sheet target in app/test/Makefile? Please give the new tests a less generic prefix than the existing tests (e.g. test-sheet-subcommand-N rather than test-sheet-N), as the various sub-targets are starting to get lengthy and we'll want to make it easier for ourselves to move them into a separate app/test/sheet/Makefile (though that move can wait until after this is done).

[br1tney5pear5]

@liquidaty 4 basic test-sheet-subcommand were added