Add external format validators option

[abogosyan]

The idea is similar to external_validator option

This option allows you to define own string formats and validate it with any fun(binary()) -> ok | error.
For example:

Validators = #{
   %% validates "IPV4:PORT" string, like "127.0.0.1:5050
   <<"ipv4_and_port">> => fun ipv4_and_port_validator/1
}, 
JesseOptions = [{external_format_validators, Validators}],
jesse:validate_with_schema(SchemaJson, ConfigJson, JesseOptions)

And somewhere in schema:

properties": {
    "str_key":   { "type": "string", "format": "ipv4_and_port" },
}

[seriyps]

Thanks for your PR! I Added some notes. No "critical" issues, just something to discuss.

[seriyps]

Oh, sorry, I completely forgot about two other things:

• would you mind adding a few tests for this feature?
• it would be also cool to mention it in a README

[abogosyan]

Oh, sorry, I completely forgot about two other things:

• would you mind adding a few tests for this feature?
• it would be also cool to mention it in a README

Will do, when i have free time :(

[abogosyan]

Finally, I've added few tests and little readme, also fixed linter errors and made compatible for erlang versions without maps support, please check.