Allow inline styles to be rendered with * or _

Some people prefer the __text__ over **text**
This pull request adds an option to implement these changes.
Fix typo
Fix another typo and make sure all tests pass
Add tests for the underscored version of the inline styles
Add bold_style and italic_style options
Tidy header case
Document bold_style and italic_style
Add test for both italic and bold using underlined style, bump version