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.
Remove extraneous else from header include
Use skip_style option to include or ignore the default stylesheet
Revert "Use skip_style option to include or ignore the default stylesheet"

This reverts commit 3742bb8a759319247d666f7d5bd2a41c5dfa87e6.
Move header below stylesheet
Merge pull request #23 from jimhester/header

Add header option to markdownToHTML