Plugin type to allow multiple elements to be declared in a single .bst file

In order to support reusability of more advanced constructs, we need a way to support express multiple elements in a single .bst file, so far the best idea for this is to introduce a new Plugin subclass allowing third parties to implement a Composite plugin type (or a differently named thing), which has it's own YAML configuration surface and allows expressing multiple elements which might have implied dependency relationships with one another.

This was discussed a long time ago in the context of "testing elements", and elaborated in a Manchester BeaverCon, and has recently been brought up again.

Note that this is on the 2.0 milestone list for now but can be removed from the milestone:

• If a well thought out proposal is brought to the mailing list and linked to this issue
• If it is agreed that an implementation of the proposal is both suitable, and can be done without breaking API.