Get rid of `github.com/jessevdk/go-flags` dependency

[EugeneOne1]

Currently, we're using github.com/jessevdk/go-flags module to handle configurable options in CLI tool but it provides some notable restrictions and design issues. For example, in #170 which adds the .yaml configuration file support we've came across the inability to properly combine the parsed options' values with values from the file.

Moreover, the functionality provided by go-flags doesn't actually justify abandoning of flag from standard library. Since we strive to keep unnecessary dependencies to a minimum, the one should be removed.

[ainar-g]

@Potterli20, see https://github.com/AdguardTeam/dnsproxy/blob/master/config.yaml.dist for an example.

Also, please keep questions not relevant to the subject to a minimum. Thanks.