[MRG] add --from-file option to sourmash sketch commands

[ctb]

This PR adds sourmash sketch dna|protein|translate --from-file list_of_files.txt.

Per @klamens request here, #1338 (comment)

Checklist

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[ctb]

Ready for review @bluegenes!

[codecov]

Codecov Report

Merging #1362 (0f83830) into latest (4f43288) will increase coverage by 5.33%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #1362      +/-   ##
==========================================
+ Coverage   88.80%   94.13%   +5.33%     
==========================================
  Files         123       96      -27     
  Lines       18207    14650    -3557     
  Branches     1405     1409       +4     
==========================================
- Hits        16169    13791    -2378     
+ Misses       1800      621    -1179     
  Partials      238      238              

Flag	Coverage Δ
python	94.13% <100.00%> (+0.02%)	⬆️
rust	?

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Impacted Files	Coverage Δ
src/sourmash/cli/compare.py	100.00% <ø> (ø)
src/sourmash/cli/index.py	100.00% <ø> (ø)
src/sourmash/cli/lca/index.py	100.00% <ø> (ø)
src/sourmash/cli/sketch/dna.py	100.00% <100.00%> (ø)
src/sourmash/cli/sketch/protein.py	100.00% <100.00%> (ø)
src/sourmash/cli/sketch/translate.py	100.00% <100.00%> (ø)
src/sourmash/command_sketch.py	85.02% <100.00%> (+0.75%)	⬆️
tests/test_sourmash_sketch.py	100.00% <100.00%> (ø)
src/core/src/lib.rs
src/core/src/signature.rs
... and 25 more

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[bluegenes]

any reasoning behind using this syntax vs load_file_as_signatures?

[ctb]

hah! no, not really, I just wrote these tests before load_file_as_signatures was available, and since it's internal test code I'm not as worried about using the public API.

[bluegenes]

nice! file lists are so handy