support for bgzipped and tabix-indexed map files

[aryarm]

my impression is that reading map files is already really fast, but this could be one option to make it faster (especially when running on it on specific regions) and reduce the amount of space required on disk

unfortunately, map files are currently space-delimited, so we would have to somehow add support for tab-delimited map files since tabix only supports tab-delimited files (I think)

[aryarm]

we can also add support for indexing them via the index subcommand