static build helper

This release comes with helper to create static binaries for hts-nim projects or scripts.
It uses this docker image with musl to have a static libc.
It currently does not support libcurl (so no https/https urls for bams) but it should otherwise work.

Usage is to take the hts_nim_static_builder binary below and run it like:

hts_nim_static_builder -s vcf_cleaner.nim --deps "hts@>=0.2.7" --deps "binaryheap"

or with a nimble file:

hts_nim_static_builder -s ../slivar/src/slivar.nim -n ../slivar/slivar.nimble

That will pull and use the musl-hts-nim docker image and to build the local code and create a static binary.

The hts_nim_static_builder is itself a static binary built using, um, itself, so it should be portable across linux setups.