Add support for a config.override.xml to the config system #3

The config system ccan currently only read from one file. It would be useful to have a second file config.override.xml which could be gitignored and would allow the user to "override" values.
This would be useful in the cas of API keys and the like. For example: you could put the production key into the main file and a local dev key into override. Or put a dummy key in the main file and the real one in the override to avoid checking in a key at all.

Steps to implement:

IConfiguredMember will need an Override() method which will set the value as overridden
IConfiguredMember will need an IsOverridden property
The Save Logic will need to check if a value is overridden or not and get the correct non-override value for the main file