Allow standalone taxonomic assignment via insect

[mhoban]

Spun off from #69, a feature request from @cajwalsh.

Expand --standalone-taxonomy to do insect assignment too. The way this will probably work is that the --standalone-taxonomy option will now get an argument, with the acceptable values of lca and/or insect. The pipeline will then just do whichever ones are asked for. If you want to run both, make the option a list in a params file.

Although I suppose we could just do --standalone-lca and --standalone-insect (perhaps as aliases for --standalone-taxonomy lca/insect?)