Added _is_setext_header(text) method
Made _is_setext_header public
Refactored to use header_utils

Used to check if a line is a setext header.
Changed when horizontal rules are converted

Now horizontal rules must be on their own line and wrapped by blank
lines to be converted. This was done to get around translated rules
being interpreted as H2 setext headers. Specifically, when converting
to all dashes and the previous line is text.
Added balancing setext style headers