Colorize lines in the post window.

Allow custom colorization of some lines in the post window. Line markers include:
ERROR:
WARNING:
-> (for the string result of code evaluated in the interpreter)
*** (usable for emphasis)

The intention was to make sure any changes in posted text would still be usable / useful to non-IDE clients without color, and to keep it minimal and informative, in the interest of aesthetics. Colorizing is only done on the first line of an error / warning - doing multiple lines was ugly, hard on the eyes, and often spilled over into unrelated text.