Consider filename extensions (where necessary) in format detection

[jmarshall]

Fix the issue of some BED files being misrecognised as FASTA/Q indexes, by — as suggested in #1085 (comment) and #1085 (comment) — adding another API function that also considers the filename extension.

I have not implemented “hts_detect_format() should only recognise files as fqi_format or fai_format if they are uncompressed” — as suggested in #1347 (comment) — as it would mean that an affected BED file would be recognised as BED if compressed but as a FASTA/Q index if not compressed, and that seems like an unfortunate inconsistency. (The inconsistency of misrecognising an actual FASTA-IDX as BED when it's read from standard input (uncommon for index files!) is comparatively minor.)

• FASTA/Q indexes (uncommon for hts_open()) are a particular case of 5/6-column BED files (comparatively common). We don't want to misrecognise any actual BED files as FASTA/Q indexes, so require a .fai/.fqi extension for the latter — which are unlikely to appear on standard input anyway, so filenames will usually be available.

We now have the machinery to recognise GZI indexes as well:

• GZI indexes have not previously been recognised, as they have no magic numbers. They can now be recognised by their .gzi extension.

Fixes #1085, fixes #1165, and fixes #1347.