clic.region.metadata: Tag metadata.* regions

Add metadata.* tags to regions.

metadata.title / metadata.author regions

If there are 2 lines at the start, with an empty line after, we treat this as a title / author combination:

>>> run_tagger('''
... Fly Fishing
... J R Hartley
...
... INTRODUCTION.
...
... Fly Fishing: Memories of Angling Days, also published as Fly Fishing by
... '''.strip(), tagger_metadata)
[('metadata.title', 0, 11, None, 'Fly Fishing'),
 ('metadata.author', 12, 23, None, 'J R Hartley')]

Anything that doesn’t match this gets ignored:

>>> run_tagger('''
... Fly Fishing
... J R Hartley
... INTRODUCTION.
...
... Fly Fishing: Memories of Angling Days, also published as Fly Fishing by
... '''.strip(), tagger_metadata)
[]

clic.region.metadata.tagger_metadata(book)

Add metadata.* tags to regions