#406 Allow tide-index to handle selenocysteine

[William S Noble]

Basically selenocysteine should react with the same thiol blocking groups as cysteine. I'm assuming that we can just add +57 to U.

This change has already been made to Comet:

http://sourceforge.net/p/comet-ms/code/886/

Here is a relevant paper:

http://dx.doi.org/10.1021/acs.jproteome.5b01028