add_sequence method for MinHash

[phiweger]

I think the API would benefit if instead of

mh = MinHash()
mh.add_sequence()
mh.add_protein()

one could type

mh.add_sequence(..., molecule='dna/protein/rna/...')

I encounter this inconvenience when directing user input from a CLI.

[ctb]

hmm, ok. on a look through the source code this doesn't seem to be problematic, although I worry a bit about the default behavior of sourmash compute in ignoring non-ACGTN characters for DNA - what happens if we try to add a protein FASTA file to a DNA minhash?