Update man page with db_xref field

Signed-off-by: Michael Barton <mail@michaelbarton.me.uk>
Add features for adding db_xref data to annotations

Signed-off-by: Michael Barton <mail@michaelbarton.me.uk>
Implement single db_xref attributes

Signed-off-by: Michael Barton <mail@michaelbarton.me.uk>
db_xref is a default gff mapping option

Signed-off-by: Michael Barton <mail@michaelbarton.me.uk>
Merge branch 'develop' into feature/dbxref

Conflicts:
	features/table/cds_entries.feature
	lib/genomer-plugin-view/gff_record_helper.rb
	spec/genomer-view-plugin/gff_record_helper_spec.rb

Signed-off-by: Michael Barton <mail@michaelbarton.me.uk>
Merge branch 'feature/dbxref' into develop