Add support for underline-style headers
Add support for underline-style headers

babbage/MMMarkdown,gonefish/MMMarkdown,isaacroldan/AttributedMarkdown,1aurabrown/MMMarkdown,jonesgithub/MMMarkdown,mdiep/MMMarkdown,isaacroldan/AttributedMarkdown,albertbori/MMMarkdown,isaacroldan/MMMarkdown,mobilejazz/MMMarkdown,isaacroldan/MMMarkdown,gonefish/MMMarkdown,AgileBits/MMMarkdown,cnbin/MMMarkdown,mobilejazz/MMMarkdown,AgileBits/MMMarkdown,iosdev-republicofapps/MMMarkdown,isaacroldan/AttributedMarkdown,babbage/MMMarkdown,isaacroldan/AttributedMarkdown,jonesgithub/MMMarkdown,mdiep/MMMarkdown,mjzac/MMMarkdown,isaacroldan/AttributedMarkdown,iosdev-republicofapps/MMMarkdown,cnbin/MMMarkdown,mjzac/MMMarkdown,albertbori/MMMarkdown,1aurabrown/MMMarkdown,albertbori/MMMarkdown