MRG: add pretty printing option to manysearch command

[ctb]

This PR adds an option-P/--pretty-print to manysearch that uses the code in #380 to produce nice output.

The default is to pretty-print, which is maybe not great if you're generating thousands of results. But it's easy to turn off with -N.

This PR also adjusts manysearch so that the average_abund, median_abund, and std_abund are not present when abundance is not available, which follows practice elsewhere in the code base.

Questions:

• should we add this to pairwise a la sourmash compare, too?
• same question for fastgather a la sourmash gather...

Fixes #379
Closes #377 because it adds an explicit test for the output.

TODO:

• add tests

[ctb]

@sourmash-bio/devs read for review & merge!

[bluegenes]

lgtm!