FIX: Correct parsing for Setext-style header underline

According to John Gruber's spec and the Stack Overflow parser, the underlining of H1 / H2 using equal signs / dashes requires 1+ instances (not 3+), and the line must not contain anything else.
Merge pull request #4 from inkarkat/fix-header-underline-parsing

FIX: Correct parsing for Setext-style header underline