cargo: shortcut for adding a source

[graydon]

It's a minor UI convenience, but "cargo add-source " is a nice way to save people the activation-energy of manually editing a json file.

[graydon]

for that matter, maybe invert the command-name and support "cargo source add ...", "cargo source list" and "cargo source rm" if you like. Basically the UI affordances given to git 'remotes'.

[eholk]

According to z0w0, this was added a while ago...

[z0w0]

In 9187406da5ce8a6b4bfca6ee017e56230be0872c.