New command to list pathogen repos #510
Description
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.