Add support for numbered headings

[DynaSpan]

This is a updated version of PR #76

Changes compared to previous PR:

• Regex changed, it now only captures the numeric group at the beginning of the string
• Supports up to 8 levels deep (as it uses EnsureMultilevel now)
• Moved the numbering logic from HtmlConvert.ProcessTag to NumberingListStyleCollection
• Fix some LINQ formatting
• Name of the heading list is now a compile-time constant

I've tested this with a document which uses diverse numbered headings and other (un)ordered lists, I couldn't spot any problems so far.

[onizet]

This is very good job 👍

[XavClevLab]

Hi, Thanks for the job!

When will the release be available? Because I admit, I need the numbering 😄 We could have a little tutorial on how to use it

Thank you!