Addition of 2 new options: sortChildren and sortInPlace

A few changes I had to implement for a project, in order to enable sorting of two-dimensional tables.

**sortChildren** - allows child rows to be sorted along with their parent
**sortInPlace** - allows visible children to be sorted, without changing the order of the parent rows. extremely useful, when your attention to the  


**Demos available at:**
http://mbezhanov.github.io/tablesorter/example-sort-children.html
http://mbezhanov.github.io/tablesorter/example-sort-in-place.html

These demos are very similar to what we use in the project I made the changes for.