Support configuration merging

[chuckwondo]

The maap.maap.MAAP expects to find a maap.cfg file in one of the following locations:

• current directory
• user's home directory
• location specified by MAAP_CONF environment variable

Unfortunately, it uses only the first one it finds, making it impossible to simply override a subset of configuration values (not even a single override). This prevents setting up a maap.cfg file with defaults, and then simply providing an additional maap.cfg file containing only overrides.

Since ConfigParser is used, "merging" is already supported, so the code changes required should be minimal.

Further, implementation of this should be accompanied by clear documentation on supported file locations, as well as the order of precedence, such that it is clear as to which location has lowest precedence (and thus may be used for "default"), and which other locations have increasing precedence.