Filter out sequences that contain unknown amino acids

https://jira.gsc.wustl.edu/browse/AT-623
For my own edification/review (and because this was the meat of the PR):

Previously, this was allowing any sequence string as long as it contained only the letters A-Z.

Now, it will allow sequence strings only if they consist entirely of one of the 20 characters listed. 

I checked the JIRA ticket (AT-623) that spawned this PR and the 20 amino acid codes listed here seem to match the 20 that Jasreet provided as a whitelist, so this looks good to me!

+1