Cstag

- add `cstag.call` to generate a cstag

- update `to_html`: visualize `N` at the beginning

- update `to_html` to return an objects, not a file.
- tidy up tests

update `test_html` to compare objects, not files

Tests on Windows

Add `CSTAG` arg to handle softclips.

python
def test_softclip():
CSTAG = "cs:Z:=ACGT*ac+gg-cc=T"
CIGAR = "2S5M2I2D1M"
QUAL = "AAAA!!!!AA"
assert cstag.mask(CSTAG, CIGAR, QUAL) == "cs:Z:=ACNN*an+ng-cc=T"