add a '--rna' option

[ctb]

Per #151 this adds --rna as a synonym for --dna.

• 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]

@taylorreiter a review would be appreciated, at your leisure :)

[codecov]

Codecov Report

Merging #585 into master will not change coverage.
The diff coverage is 83.33%.

@@           Coverage Diff           @@
##           master     #585   +/-   ##
=======================================
  Coverage   88.64%   88.64%           
=======================================
  Files          25       25           
  Lines        3786     3786           
  Branches       37       37           
=======================================
  Hits         3356     3356           
  Misses        430      430

Impacted Files	Coverage Δ
sourmash/sourmash_args.py	95.63% <100%> (ø)	⬆️
sourmash/commands.py	89.33% <66.66%> (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 4ccdd3d...4289c7c. Read the comment docs.

[ctb]

Ready for review and merge! (The red flagged coverage differential is not real, it's due to some outdated information.)

[luizirber]

codecov/patch is not required anymore (I still think it's a good idea to keep codecov/project as a required check)

[ctb]

bump!

[taylorreiter]

looks good to me!