bring back code for syntax highlighting

meet org.nlogo.compile.front.Colorizer.  it brings back to the
headless branch the code-colorizing abilities that were once the
domain of app.ProceduresToHtml and window.EditorColorizer
in colorizing code, use up-to-date terminology

bring naming in line with current practice ("literal" not "constant")
beef up ColorizerTests a little

test an additional token type I forgot before
Merge pull request #500 from NetLogo/headless-colorizer

headless branch: bring back code for syntax highlighting
Merge remote-tracking branch 'origin/headless' into tortoise