support import of gff2 alignments from exonerate coding2genome and cds2genome

Exonerate's cds2genome and coding2genome modes align transcripts and CDS sequences to their locus. The GFF export provides the alignment and (assuming the correct --showtargetgff/--showquerygff option was provided) annotation on the locus identifying exons and splice sites (in the case of 'coding2genome').

Import of GFF2 files with sequences appended (after ##FASTA) doesn't seem to work reliably for these modes and the GFF2/3 parser for 2.11.1 includes comments suggesting only protein2genome is currently supported.