Added support for files which make use of the GFF3 FASTA directive

This PR allows gff2bed.py to recognize [GFF3 ##FASTA sections](http://www.sequenceontology.org/gff3.shtml) and stop processing without emitting any error messages.

Currently, once the script encounters the first entry in the ##FASTA section it fails to parse the line and emits and error message ("Error: Could not import GFF data...")

Example gff file to reproduce the issue: [TriTrypDB-7.0_LmajorFriedlin.gff](http://tritrypdb.org/common/downloads/release-7.0/LmajorFriedlin/gff/data/TriTrypDB-7.0_LmajorFriedlin.gff)