Fix #44, #46: roff: Clean up unnecessary dot escaping, fix dot escaping in NF environments

[sometimesfood]

This moves dot escaping at the beginning of a line from line to node level.

Fixes #44 and #46.

There's probably a better way to do this, but I figured it was at least cleaner than depending on implementation details in the write method which would have been another naïve solution.

[sometimesfood]

@rtomayko: Any news on this? Just rebased my feature branch to the current master.

@thinkerbot provided a different fix for issue #46 in pull request #65, you might want to check that out, too.