add support for doxygen's \mainpage

[bearsh]

currently the \mainpage content doesn't end up in the markdown output. it would be nice to have all the doc in the source files...

[tonykero]

I did want that too. I successfully got rendered pages but, it worked great, and i don't know why i am getting newlines in tables.

tonykero@dcbd271

[auscaster]

Thanks for the PR @tonykero, just merged!