Provide an index of TODOs

[machawk1]

Akin to the Figures, Tables, etc. indices at the beginning of a document, it would be helpful to have an index of all of the TODOs in the document. As far as I can tell, they are not currently indexed using the current mechanism but maybe we could build this index into the TODO command and provide this index as a sort of checklist to the writer.

[ibnesayeed]

This is a very good idea. It should print the table of TODOs in the revision mode and print a warning in final mode.