Blocky Banana

Adds a custom keymapping that I (@johnfn) have always wanted. Press af in Visual Mode to select ever-increasing blocks of text.

e.g. if you had "blah (foo [bar 'ba|z'])" then it would select 'baz' first. If you pressed az again, it'd then select [bar 'baz'], and if you did it a third time it would select "(foo [bar 'baz'])".