fix for https://code.google.com/p/k9mail/issues/detail?id=6186

Only convert =, -, and _ characters to an <hr> if they are the only chacters on the line and if fixed width text is not turned on.
Only quote indent lines if the first character is the '>' character.
Fixes https://code.google.com/p/k9mail/issues/detail?id=6186