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.
This results in those three colors taking the default background color of the document view, or the background of the "text" color, if it's set (it's also not by default).