Add options to set DHCP range/scope

[DevinKott]

Is your feature request related to a problem? Please describe. I'm in need of configurable DHCP settings; specifically, configurable IP ranges. Current DHCP settings seem to be generated on the fly in the /tmp directory, in the dnsmasq.conf file, and are not configurable by the consumer of the software.

Describe the solution you'd like I see the dnsmasq.conf file generated on/around line 1747 (as of Aug 17th). The following are options I've considered.

1. Option 1 would have configurable lower and upper IP variables. Right now these are hardcoded as 1 -> 254 on line 1747.
2. Option 2 would be something like a "start IP" and then the number of clients you want. For example, start at 150 and allow only 10 clients (.150 -> .160).
3. Option 3 might be a custom dnsmasq.conf file you pass in yourself to be used. This would require more work, as I'm not sure what DHCP options are required/used by the application already (checking would have to be added to make sure the custom .conf includes what is required by the application).

Describe alternatives you've considered I have not considered any other alternatives. This issue is just to gauge interest in submitting a PR to officially add this functionality, as I already plan to make the changes for my personal use.

Other I am most likely implementing option 1 for my personal use, but would consider implementing option 2 or 3 (or a solution someone else suggests) if others are interested in adding the functionality to the main branch.