Added rough initial code for a subsystem for running pieces of text in an external shell/command interpreter.  Uses Expect, but maybe with moving to Tcl 8.6 I can just use Tcl channels.
Merge branch 'master' of github.com:screwtop/Piaf
 - Added some code from the Tcl/Tk wiki for line numbering using a canvas down the left margin.
 - No need to report an error if there's already a word count in progress - just [return].