Makes single backstroke deleting the default behavior.

I think we should consider ripping out the double backstroke
supporting code entirely.
Update docs to reflect default change on single_backstroke_delete
Properly handle defaults on single_backstroke_delete

`@options.single_backstroke_delete || true` is _always_ true.
Spell pressing correctly.
Merge pull request #1375 from harvesthq/single-backstroke-delete-default

single_backstroke_delete defaults to true