Support for gzip'd GTF input

[nick-youngblut]

Currently, stringtie does not support gzip'd GTF input. The error:

Warning: unexpected tab character in last column, line truncated:
�O�+���9��2��e�Ih6B2�s����K��wJ�*� ��ܟ�

It would be helpful if gzip (and maybe other compression formats) were supported by stringtie.

stringtie install: v2.2.3, bioconda