magic initializer-list for HTTP ABNF

[vinniefalco]

Often it is desired to specify a list of elements from the HTTP RFC ABNF. For example, consider this

cors.allowedHeaders = "Content-Type,X-Referred-By"

It would be better if we could write

cors.allowedHeaders = { field::content_type, "X-Referred-By" };

There is a prototype of this system here
https://github.com/cppalliance/http_proto/blob/develop/test/unit/server/cors.cpp#L94

It needs to be developed into a complete solution. Custom strings that do not pass validation should cause an exception to be thrown.