Better mapping management on the root domain

[svandragt]

On a multisite, when trying to manage domains on the root domain (when no --url is specified) the following error message is returned:

Error: Domains cannot be mapped to the main / root Site.

My expectation is that in this scenario instead of the error message, domains on all sites are considered. As it is not possible to assign the domain to multiple sites, it should be irrelevant what site is specified, when managing existing mappings.

The exception to this is when adding a domain and no site is specified, not sure if an interactive prompt would be useful there or if the error message could be clarified to mention how to prevent the error condition.

[cameronterry]

Added a note here that this issue is loosely related to #8. Solving one would likely form the foundation to solve the other.