Implements support for specific build-config and recipe-append files

This MR provides a solution to bob/bob.conda#71 by allowing developers to provide branch-specific conda_build_config.yaml and recipe_append.yaml files.

All ci scripts and functions will now search for a branch specific files conda_build_config-my-branch.yaml and recipe_append-my-branch.yaml on first on each recipe directory, then on the current directory. If any of those are found, then use these instead of the defaults to build the said package. If they are not found, search for files with names conda_build_config.yaml and recipe_append.yaml on both the recipe dirs, then on the current directory. If any of those are found, they are used. Otherwise, if no specific build variants or recipe-append files are found, use the defaults shipped with this package.

Closes bob/bob.conda#71