Add project level configuration file

[jamesroutley]

For tool-level config. To allow specification of the tools behaviour at per-machine level.
Config may be stored at ~/.sceptre/config.yaml
Example config could include:

• Default output for Sceptre - i.e. colour, no-colour, verbose
• Disable requirement -y flag for commands
• Default AWS profile to use
• Specify default location to search for external templates.

[zaro0508]

we already have a place for project level configs, it's in config/config.yaml