Better markdown conversion control

[neocotic]

Update html.md to the latest version and add controls (i.e. options) for the conversion. For now, at least, these controls will be limited to the options that are provided by the library itself on a new markdown option which will be stored:

• absolute - All copied URLs (e.g. for links and images) will be absolute
  • Extra care should be taken here since the library is called within the context of the background page, it's likely that this will resolve based on the background pages URL (additional parsing may be required)
• inline - Generate links in Markdown using the inline style (include examples in description)

These new options will be accessible in a new Markdown section of the General tab on the options page. A brief description should be provided explaining what the options relate to.

The new options should also be made available as template variables (e.g. markdownAbsolute).

[neocotic]

I'm not sure absolute is a valid option any more since I just remembered that content.coffee already forces all links and images within the user selection to have absolute URLs.

This is probably still worthwhile for inline though.

[neocotic]

This functionality has been added by PR #188.