Support omitting "start" and "stop" to obtain all data for a given reference sequence

[nathanweeks]

Currently, when invoking the underlying pysam object's fetch() method, fasta-api requires start and stop coordinates to be specified; e.g.:

@app.get("/alignment/fetch/{contig}:{start}-{stop}/{url:path}")

To obtain all, e.g., alignments for a given contig, the "start" can be hard-coded to 1, but the user still needs to issue a separate query to determine the length of the contig(s) of interest to determine the "stop" value to enter.

pysam supports omitting start & stop to mean "the entire contig"; it would be convenient if fasta-api supported this as well; e.g.:

@app.get("/alignment/fetch/{contig}/{url:path}")