MRG: update plugin support for argparse usage/description/epilog

[ctb]

This PR adds support for argparse usage and epilog strings, as well as formatter_class, per argparse docs.

See sourmash-bio/sourmash_plugin_template@cb5d454 for general use in plugins, and ctb/sourmash_plugin_commonhash@3fbb4d2 for an example use in commonhash.

This means that e.g. commonhash now supports pleasing output for both bad usage and -h flag -

sourmash scripts commonhash    
usage: 
   sourmash scripts commonhash [ -m 2 ] *.sig.gz -o filtered.zip
 commonhash: error: the following arguments are required: sigfiles, -o/--output

and

== This is sourmash version 4.8.3.dev0. ==
== Please cite Brown and Irber (2016), doi:10.21105/joss.00027. ==

usage: 
   sourmash scripts commonhash [ -m 2 ] *.sig.gz -o filtered.zip

filter hashes by min occurrence across sketches

Reduce 'noise' in Jaccard comparisons of sequencing data by removing
hashes that are present in only a few sketches.

'commonhash' supports the full range of sourmash sketch output formats [1].

[1] https://sourmash.readthedocs.io/en/latest/command-line.html#choosing-signature-output-formats

---

positional arguments:
  sigfiles              input signatures

options:
  -h, --help            show this help message and exit
  -q, --quiet           suppress non-error output
  -d, --debug           provide debugging output
  -k KSIZE, --ksize KSIZE
                        select this k-mer size
  -o OUTPUT, --output OUTPUT
                        save sketches to this location; e.g. output.zip or ./output/
  -m MIN_SAMPLES, --min-samples MIN_SAMPLES
                        a hash must be in this many samples to be retained

See https://github.com/ctb/sourmash_plugin_commonhash for more examples.

Need help? Have questions? Ask at http://github.com/sourmash/issues!

[codecov]

Codecov Report

Merging #2696 (6cb3b20) into latest (02d2331) will increase coverage by 7.57%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #2696      +/-   ##
==========================================
+ Coverage   85.26%   92.83%   +7.57%     
==========================================
  Files         133      104      -29     
  Lines       15182    12401    -2781     
  Branches     2612     2614       +2     
==========================================
- Hits        12945    11513    -1432     
+ Misses       1936      587    -1349     
  Partials      301      301              

Flag	Coverage Δ
hypothesis-py	25.80% <9.09%> (-0.02%)	⬇️
python	92.83% <100.00%> (+<0.01%)	⬆️
rust	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
src/sourmash/plugins.py	100.00% <100.00%> (ø)

... and 29 files with indirect coverage changes

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