Introduce mixtool generate command

[metalmatze]

Recreate the generate command. As per Twitter poll we now call this command generate.

Using this command looks like:

mixtool generate alerts mixin.libsonnet > alerts.yaml
mixtool generate rules mixin.libsonnet > rules.yaml
mixtool generate dashboards mixin.libsonnet --directory /tmp/dashboards

The first two command simply write to stdout and you can either pipe it somewhere or write to a file. Generating dashboards needs to get a directory path via flag, as we're writing multiple files.

What do you think @brancz @tomwilkie?

I'm actually testing these commands while continuing development on the Thanos mixin.

[brancz]

lgtm