Remove the dependency on configobj and have default values for all
configuration settings. User settings are defined in a Python module
pointed to by the MUTALYZER_SETTINGS environment variable.

We also clean up many configuration settings and remove some that
are no longer used.

We no longer look for /etc/mutalyzer/config and ~/.config/mutalyzer/config
since it is too low level and inflexible. The user should now specify the
location of the configuration file in the MUTALYZER_SETTINGS environment
variable (or the file mutalyzer.conf in the current directory is tried).