Add optional Hunter support in cmake buildsystem

[Bjoe]

Few months ago, I try to use Poco with conan but without much success. Then I found Hunter
Since that I use/build Poco with Hunter and I also build a POC for macchina.io macchina-io/macchina.io#71
Poco was also used in Hunter before (PocoCPP) but they patched "Poco cmake build system." See also here: https://github.com/ruslo/hunter/pulls?utf8=%E2%9C%93&q=is%3Apr+is%3Aclosed+PocoCPP

Hunter is only optional and as default it is off. So the current cmake build system is not affected. If you switched it on with the cmake parameter:

cmake -Hpoco -Bbuild-poco -DHUNTER_ENABLED=ON

First time, it will take a time, because hunter is building the third-party libraries. After third-party are build, it is only building Poco.
With Hunter you can add Poco easily in your project without installing any new "dependencies". Hunter is based on cmake only.

[aleks-f]

@Bjoe if you still want this in, it needs to go against devel branch