Multipass installation

[angrybrad]

See if it's possible to check if Multipass is installed and if not, automatically download

[jasonmccallister]

We check is multipass is found here: https://github.com/pixelandtonic/nitro/blob/develop/internal/runner.go#L66

If we are running on macOS, we could check for brew and attempt to install multipass

[jasonmccallister]

Maybe instead of downloading, we can check if they have something like brew installed and then print the brew (or suitable package manager) instructions:

e.g.

We are unable to location multipass, to install please run:

    brew install multipass

When multipass is installed, please run this command again.

[angrybrad]

If we do this, we have to link to their license and prompt the end-user to accept.