Added special target EXECUTABLE_PER_SOURCE and colorization features

[jdc1234]

To satisfy the GPL I want to feed back updates I've made for use in my own projects in case you're interested.

I added a special target EXECUTABLE_PER_SOURCE in which case each source file is assumed compile to its own executable. With many simple tools, this reduces the maintenance of having multiple .mk files and targets. This required moving portions of INCLUDE_SUBMAKEFILE to their own functions (INIT_TGT and PREP_SOURCES).

I also added some colorized output, features though these aren't critical. If you're actually interested in merging I can remove/isolate these changes.

Thanks for putting boilermake out in the open!