Use runtime config file

[The-EDev]

Allow Crow to use a config file to define anything that would be specified in crow::Crow methods or macros. This was suggested by @ipkn in a TODO:

replace with runtime config. libucl?

Although libucl is suggested, I would prefer either using a simpler (possibly single file / C++) library, or implementing our own key, value config parser.

[joelguittet]

Hello, proposition for this : using json file. I think you already have most of the tools in crow::json :-)