This puts config in its relevant location and moves functions around
loading config files into the main package.
As a side effect of removing cyclic imports for the API config, the
context library is no longer used.
Clair will now use a YAML configuration file instead of command line
arguments as the number of parameters grows.
Also, Clair now exposes a Boot() func that allows everyone to easily
create their own project and load dynamically their own fetchers/updaters.
