Add heading auto-numbering

[jmmv]

We need the ability to auto-number headings in the output. There are two use-cases for this feature:

1. Long posts. The numbering can help navigate the post and more clearly highlight where the headers are in the text. The user should be able to enable/disable the auto-numbering.
2. Posts with more than two levels of headers. Because of the limited formatting options in the Google+ output, there is no way for the reader to differentiate headers above the third level. The tool should automatically enable auto-numbering when it detects that the headers cannot be disambiguated purely by their formatting.