Add `--install` to `step ca root`

[mmalone]

Like the --install flag on step ca bootstrap, this flag would automatically run step certificate install on the root certificate in your system's trust store (but without doing any other configuration, like step ca bootstrap does).

step ca root currently take an optional positional argument, indicating the name of the file to write the root cert out to. If that argument is omitted, the root cert is written to standard out. With the --install flag it feels weird to require writing to disk / standard out. As a strawman, I propose that this flag disables any output of the root certificate. It's an error to provide a positional argument along with the --install flag, and the certificate is not written to standard out.

As extra credit: it'd be nice to also have a --uninstall flag that runs step certificate uninstall.