rbt now supports colorized logging when writing to a TTY (otherwise,

uncolored logging will be used). The colors can be specified in

.reviewboardrc in the COLORS variable, which maps logging levels

to colors (as plain text names) or None. An invalid color name will

be mapped to no color.
As part of this, the config parsing for nested (dict) variables has

been updated to be more generic and provide defaults where

appropriate.

Ran rbt to pipe and to a file: colored logging was not observed.
Ran rbt to a terminal: logging was colored appropriately.