Allow coloring contexts and projects independently from the rest of the line

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).

With this patch applied, putting something like `export PROJECT_COLOR='\033[2m'` into your .todo/config will result in projects being underlined in the todo.sh output, for instance.