Add `sourmash index` option

[olgabot]

While I'm not doing a lot of sourmash compare (see #75) I may still want to make an SBT index.

Reasons:

• SBT indices are easy to add a single new sample to, comparison matrices are not
• SBT matrices may (eventually..) be localized and fed into KNN graph algorithm using tools like UMAP (Add option for SBT creation to be localized  sourmash-bio/sourmash#925)

[pranathivemuri]

for this issue, is sourmash index done on all the sketches from peptides, nucleotides, and ska sketch results?