Reorganize configuration of assembly lines

[tobinus]

Currently, the assembly line (see #36) is configured through __init__.py inside metadata_sources, which is counter-intuitive and breaks consistency with all other appearances of __init__.py. Additionally, local changes to that file will conflict with later upstream changes of the file, since it doesn't use the template pattern used elsewhere for configuration files.

There is also an unlogical split in where configuration is done. Settings passed to each individual metadata source is written in generator/settings.py, yet activating/deactivating and moving the sources is done in __init__.py. All settings relating to the metadata sources should reside in the same location. As a bonus, we could make it possible to have multiple instances of the same metadata source with different options, so that the same metadata source can be used for multiple date ranges or using different sources (for example, you could have a generic WordPress metadata source and use it twice, with two different blogs. Currently, you'll need to copy+paste the metadata source or write a wrapper around it, since all metadata sources are initialized in generate_feed.py with the same settings for all instances of the same metadata source.

• Move configuration to obvious location
• Move settings for metadata sources together with the rest
• Allow multiple appearances of the same metadata source, with different options