Compound Textile table cell specifiers

Adds support for multiple specifiers within a single cell, for example right alignment within a header cell with `|_>.` or `|>_.` Updated tests include some very contrived cases of compound cell modifiers.

I was very glad to see that you are working on colspans and rowspans! This plugin will be very useful in the near future for updating our wiki with a few complex tables that would otherwise be unmanageable. I am glad to contribute a bit of free time to make that possible.