Added attrib type for Havana readthrough transcripts.
Added attrib type for Havana readthrough transcripts.

Ensembl/ensembl,Ensembl/ensembl,james-monkeyshines/ensembl,james-monkeyshines/ensembl,james-monkeyshines/ensembl,danstaines/ensembl,muffato/ensembl,muffato/ensembl,willmclaren/ensembl,mjg17/ensembl,danstaines/ensembl,mjg17/ensembl,danstaines/ensembl,james-monkeyshines/ensembl,muffato/ensembl,willmclaren/ensembl,at7/ensembl,mjg17/ensembl,Ensembl/ensembl,muffato/ensembl,mjg17/ensembl,Ensembl/ensembl,at7/ensembl,at7/ensembl,at7/ensembl