NCBI ingest: reduce manual curation of hosts

[joverlee521]

We currently manually curate a host_map.tsv for mapping specific host names to host groups (Avian, Nonhuman mammal, etc). We can probably reduce the manual curation of the host groupings by doing something similar to nextstrain/rabies#8.

It has not been too much overhead to curate and update this host map manually, so this could potentially wait on nextstrain/augur#1586.

[jameshadfield]

(I'd vote to wait, although I did update it only yesterday! #89)