Support gzip-compressed alignment files

e.g. `pplacer -c vaginal_16s.refpkg src/p4z1r36.fasta.gz` generates
p4z1r36.jplace
Support reading compressed .jplace files
Support writing compressed .jplace files

checked via:

    pplacer -p -c vaginal_16s.refpkg src/p4z1r36.fasta.gz -o test.jplace.gz
Remove duplicate function chop_suffix_if_present

Same as Ppatteries.safe_chop_suffix
Merge pull request #323 from matsen/323-compressed-alignment-files

read compressed alignment files?