Gtf parse

Based on existing GFF3 parser. GTF parser takes into account the differences in the GTF format. This parser also tries to account for GTF variations with both specific lines for transcript features and (following the GTF spec) where transcripts are implied by transcript_id.

Tested with Cufflinks output and the following:
ftp://ftp.ensembl.org/pub/current_fasta/mus_musculus/dna/Mus_musculus.GRCm38.75.dna.chromosome.1.fa.gz
ftp://ftp.ensembl.org/pub/current_gtf/mus_musculus/Mus_musculus.GRCm38.75.gtf.gz (chromosome 1 portion)