Implement dynamic updating of manually defined rules

[c-mueller]

Currently whitelist and blacklist rules can only be defined within the corefile. In order to change them CoreDNS has to get restarted. In order to allow a simple method to reconfigure rules dynamically the following feature should be implemented:

Rule can be defined in a sepaerate file (or maybe multiple ones) this file gets reloaded every 5 seconds or maybe through a filesystem hook (on change). The idea of this should be simmilar to the reloading mechanism of the hosts plugin.

For syntax i think a ignore file like syntax is a good approach (excluding wildcards)
A rulefile could look like this:

# A comment
!google.com # Whitelist google.com
my-domain.com # Blacklist my-domain.com
!<REGEX> # Whitelist a regex
<REGEX> # Blacklist a regex