Add feature to allow duplicate features in encoded output

Signed-off-by: Michael Barton <mail@michaelbarton.me.uk>
Implement duplicate features

Signed-off-by: Michael Barton <mail@michaelbarton.me.uk>
Merge branch 'refactor/non-unique-attributes' into develop
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>