bst-fmt: Allow greater control over order

Allow authors of plugins to utilise the Plugin.keyorder attribute to
define an order for the sub-headers in config. Modify `bst fmt` to
modify the yaml into that order. Add canonical keyorder for every plugin
already in core.

In terms of implementation, this adds a custom yaml dumper subclassed
from ruamel.yaml's RoundTripDumper, which will dump a dict in the order
defined in the keyorder attribute. The Plugin class is given a keyorder
attribute, which defines the order in which keys should be dumped. The
element and source classes add the common fields used in them, so as to
minimise work for plugin authors. Plugin authors should add their custom
keys at configure time.

This causes some stripping of comments, which seems to be due to a bug
in the ruamel.yaml RoundTripDumper, as that also strips the same
comments. It also will remove blank spaces, again probably due to
limitations in ruamel.