Added strict subdomain check, domain option and tests.

Including the following enhancements and fixes:

* Added conditions to the regex to ensure the ```base``` string matches only at the beginning of the requested url and before any ```.```. This prevents the subdomain string matching any part of the url, i.e. in the middle. Tests added.
* Added an option to set a ```domain```, so you can check for the full url (```base``` + ```domain```) and not only the subdomain. This is fully optional. Tests added.
* Removed the parameters on the call to ```next``` in the middleware function. That was breaking Express. Tests updated.
* Minor linting changes.

Thanks for sharing this module!!!