Option to disable generation of Table of Contents on Markdown input

[david.woods]

It's not always appropriate to create a table of contents (e.g. on short stories).

* Added option to TXT input dialog to enable/disable creation of TOC for markdown input

* Added Option Recommendation? to txt input

* Modified txt processor to pass option to markdown processor

* Modified markdown processor to handle extension config

* Modified markdown TOC processor to skip TOC if required

[kovidgoyal]

Implemented in branch trunk. The fix will be in the next release.