Basic json initialization has been implemented.

Formatter and sink config holders now provide const operator [].
Decomposing json configs into separate files.
Added asserts about parsing errors.
Integer conversion issue has been resolved.

Now it’s necessary to register exact integer type (i.e. std::uint16_t for port) when you have variable with not `int` type.
Multiple fronted test case has been implemented.
Refactoring over tests.
Refactoring over json parsing class and methods.
Namespace decomposition.
Package decomposition over config parser classes.
Rejected idea with integral conversion, because it leads to bad code.

Therefore, I should think about simplifying configuration from file.
Integer conversion support added.

Suppose we read config from json file for socket port and get some integer value (let it be 42).
We know, that socket config class keeps port variable in `std::uint16_t`, but json parser doesn’t know that!
It writes value to the intermediate config transport (boost::any at this moment) as usual 4-byte int.
See, we have connectivity problem - extracting `std::uint16_t` from boost::any will lead to the exception.
To suppress this behavior, I’ve introduced supported integers vector over which we will iterate and try to
extract when `boost::any` config received.

This problem is only for integers until you store float-point numbers in `float`. If you are - then we need to implement
another `cast_traits` template specialization, which is not hard.
Drop conversion integral entity. Horaaay!
Subtask added.
Added example with initializing from file.