ENH: Add nextstrain update --all (or all) command or support multiple arguments

[corneliusroemer]

Context

I have multiple nextstrain runtimes installed. It would be nice if there was a way to update both conda and docker with one command.

This could be done by supporting all or --all. Or by allowing multiple runtimes to be passed to update, instead of just one.

Maybe passing multiple arguments would be easier to do, just need to change argparse in one place.

Something like this:

[tsibley]

I'd favor nextstrain update accepting multiple runtime names, just like nextstrain check-setup does. Without any arguments, the behaviour should still be to update just the default runtime. Besides changing the argparse parameter in nextstrain.cli.command.update.register_parser(), we'd also need to update nextstrain.cli.command.update.run() to handle multiple runtimes sensibly. This includes aggregating final status checks so an appropriate exit code can be used even if some runtimes are successfully updated and others are not.