Improve conditional extractors

[lukavdplas]

It's not uncommon for extractors to be conditional, i.e. they should return None unless some condition holds.

The extractor class supports a condition applicable which allows you to check a property in the metadata, but this is not very flexible. It only works on the document level, for one thing.

It would be nice to change this to a more general condition, where applicable instead takes an extractor.

For the old functionality, you can use the Metadata extractor:

#extract the foo tag if 'bar' is set to True in the metadata
XML('foo', applicable=lambda metadata: metadata['bar'])

# becomes:
XML('foo', applicable=Metadata('bar'))

However, other cases can now use the same syntax to be a lot more readable:

# extract the foo tag if a bar tag exists in the node & is truthy
Combined(
	XML('foo'),
	XML('bar'),
	transform=lambda values: value[0] if value[1] else None
)

# becomes:
XML('foo',
	applicable=XML('bar')
)

[lukavdplas]

Note: the secondary_tag property on the XML extractor is also used to specify some kind of condition which could be captured by this as well, I think. -> this already covered by #18