Drop hard-coded dependency on ts=4.

It was just a small matter of s/'    '/repeat(' ', &ts)/.
Properly treat empty lines.

A simple check, IsEmptyLine() && IsFollowedByHeading(), restores
Vimoutliner's behaviour: trees can now be grouped and separated by
whitespace in wretch.
Update documentation.

Rename the README and fix appearance on github.  Update design.wt with
latest successes.