New command to list pathogen repos

[victorlin]

Context

Currently, a list of locally set up pathogens can be shown with nextstrain version --pathogens. This feature is a not very obvious, non-default behavior of nextstrain version, and is somewhat limited by name.

Proposal

Move the functionality to a new command, tentatively named nextstrain list. For starters, it can show the same output as nextstrain version --pathogens. It can be later expanded to include GitHub repo links or list pathogen repos available for setup.

[jameshadfield]

Bumping this (I think it was me who originally asked for it!) -- I was reaching for this today, I knew the command existed but the only place I could track it down was this issue!