add key tool

[phish108]

in addition to the *key tools, a more mnemonic version of the tools would be nice, so one can call

jose key new -u sig -R -s 2048
jose key add -j keystore
jose key ls -j keystore 
jose key rm -j keystore 
jose key find -j keystore some_kid
jose key export -j keystore --format pem
jose key publish -j keystore

This requires the sanitizer to check more than one arguments.

Maybe it even makes sense to split the tools as such:

[phish108]

Decision: The key tool is ok, tool shorthands are not.