Components should not have magic words like docs

[simonrainerson]

As discussed in #288 components should not give special meanings to attribute names.

One solution mentioned in the PR is:

We could do something like tagging everything created with the base.documentation functions (and add a base.documentation.includeInCombinedDocs function to tag custom derivations), and then do all the special stuff with everything in the component that has the tag. Then it wouldn't matter where in the component they were so it would be a bit flatter.

Another solution would be to add a config setting for what the docs attribute should be called.

We should probably also try to be more consistent about using the config for default target, right now we hardcode package in some places and use the config value (which defaults to package) in other.