Added add-thought to scripts.

I figure that it's useful enough and good enough by this point to be
included.
Update NEWS.
Updated add-thought to allow subthoughts.

Only works in non-gui mode for now, since there's no easy way to tell people what subthought is which with the GUI.
Switched subthought and severity in function arg
Merge pull request #388 from Putnam3145/add-thought

Add "add-thought" script to main repo