send DNS over HTTP via cloudflare

[s5k6]

Hi, you really need to review this, but it should disable the TRR feature that is being prepared by mozilla right now. That would send all DNS queries to cloudflare. See https://wiki.mozilla.org/Trusted_Recursive_Resolver

[allo-]

Thank you for your timely pull request, which tries to solve #124.

The commit has a few minor issues, so I will create a working file from it. But I am happy for everyone contributing, so a little explanation what needs to be changed:

• label is the title of the setting, which means what will happen when you change it. Its something like "Disable DNS over HTTP".
• Your help_text is fine (I will tweak it for a boolean setting)
• type is currently boolean (checkmark), choice (choice field, have a look i.e. in the referer setting for how to use it), or text for free form text
• initial is the initial state of the setting, which would be for example true (DO disable it)