Customise bonfire without forking

[jjl]

Some members of the team work on multiple projects using this codebase and have to maintain different branches for each project. This is error-prone and we could do better.

• Config-driven routing, see Config-driven routing #88
• Load mess deps files from a directory provided in the environment variable BONFIRE_FLAVOUR
• Load compile-time config from a directory provided in the environment variable BONFIRE_FLAVOUR
• Load release config from a directory provided in the environment variable BONFIRE_FLAVOUR
• Load migrations from a directory provided in the environment variable BONFIRE_FLAVOUR