Little rewrite of config_t.

The main goal of this PR is to use dynamic_t's conversion code in config_t because:
* It's not good that there are two solutions of the same problem in one code base.
* The config_t's conversion code was hard to extend.

Also there are minor changes in the API.

Any comments about the API? Better ideas how to use dynamic_converter in config_t without that Controller kludge?

Personally summoning the author @euroelessar . Ruslan, why was there the restriction that a config cannot contain null value? I removed it, because it looks artificial and unnecessary. Does it break anything in the code or in the concept?