Config from variable or file

This replaces the old file of credentials (with one per line) with a
configuration in YAML format.

The configuration can be stored either in a file or in an environment variable
(BITTER_CONFIG).

There is still a command line argument to add the credentials in that file to
the config.

.gitignore

@@ -2,6 +2,7 @@ __pycache__
 *.egg-info
 dist
 env
+.env
 __*
 .*
 *.pyc