GFF file regex

[roldanjg]

Description of feature

Hi nf-core team,
I work with genomes from different species, and many times people use 'gff3' instead of 'gff' extension in the annotations file name, to differentiate GFF2 and GFF3 I guess. I had to modify the regex to include this possibility, so I'm about to propose a pull request to include that and so 'gff3' extension won't give a bad format error.

[drpatelh]

Linking PR here for posterity: #1191

It still needs some comments addressed, so I will leave this issue open for now.