Add registry mirror flag to start command

[gvkhna]

Description

I don't edit the template file since my colima setup is all cli usage based and pretty simple, I just set cpu and memory on start.

A start flag for adding a registry mirror entry would be great. I'm sure many folks would add gcr mirror too!

colima start --registry-mirror "https://mirror.gcr.io" --registry-mirror etc...

Thank you.