Add support for filtering rows by flags

[althonos]

Hi!

This PR adds a filter method to AlignmentFile to support reading flags that have given flags set or unset. This is faster than doing this test in a for loop in the Python code because it avoids a useless copy of reads that are discarded.

For instance, on a BAM file with 6,369,435 reads with only 315,068 of them being mapped, iterating with

for read in AlignmentFile("reads.bam").filter(flag_filter=FUNMAP):
    pass

is about 2x faster than:

for read in AlignmentFile("reads.bam"):
    if read.is_mapped:
        pass

I added some tests and type hints as well.