Add header option to markdownToHTML

This adds a header option to markdownToHTML, analogous to pandocs
-H include header option, which inserts the text or file into the
HTML header rather than the body.  This is useful if you have
custom javascript you would like to include in your final HTML
for example.

I think it is useful to have in markdownToHTML rather than using the fragment.only option and appending the html yourself as you can pass this header option when using knit2html in the knitr package.