Improve configuration system

[craftamap]

We need to add some features to improve what's possible with bb's configuration:

• Add local configurations #16
  ... like a specific merge strategy for only a specific repository, or a different username.
• Add a way to configure the remote with said local configuration #17
• Add a way to modify the configuration with commands #18
  this should propably be git-like:
  bb config [--local|--global] [--get] [value]