[MRG] allow all ksizes for protein input

[bluegenes]

Addresses #575

• 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?

[codecov]

Codecov Report

Merging #576 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #576   +/-   ##
=======================================
  Coverage   88.53%   88.53%           
=======================================
  Files          25       25           
  Lines        3786     3786           
  Branches       37       37           
=======================================
  Hits         3352     3352           
  Misses        434      434

Impacted Files	Coverage Δ
sourmash/commands.py	89.33% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8f7379c...ab66319. Read the comment docs.

[ctb]

hi @bluegenes could you add a test, please? thx! no hurry.

Please change title to [MRG] when it's ready for review.

Also, please update from latest master. Note that if you put PR branches in the dib-lab/sourmash repo (you should have permissions) then anyone in dib-lab can update from master for you.