Checks on custom URL

[jblack-vail]

For security purposes ensure that the custom API domain is validated by the user and/or a pattern

[jblack-vail]

Consider just displaying a warning if the base domain doesn't match a regular expression. Still need to figure out whether there's a way to do this without cluttering the output for each command. Could this be integrated with the existing error handling? Or is this feature not necessary after all?