RFE: Allow customisations (such as altername `origin`URL)

[RoUS]

I keep my castle repos on a private server, so every time I use homesick generate I have to go in and use git remote set-url origin to point the checkout to the correct URI.

This should be overridable by envariable, or, better yet, a config file -- such as ~/.homesick/config or $XDG_CONFIG_HOME/homesick/config. The latter approach would allow setting defaults for other commands, similar to .gitconfig.