Allow coloring contexts and projects independently

Because @contexts and +projects are metadata as much as they are text,
it can be nice to highlight them independently of the coloring of the line
that contains them.  This is tough to do correctly in
a TODOTXT_FINAL_FILTER because doing it at that layer requires
re-parsing the text of the line to find the appropriate PRI_$x color to
reset to after coloring the word, or trying to reverse engineer where
the color codes inserted by todo.sh end and the user's text begins (and
vice versa).

Closes #119