Kata

Implement String#parse_mana_cost, which parses Magic: the Gathering mana costs expressed as a string and returns a Hash with keys being kinds of mana, and values being the numbers.

Don't include any mana types equal to zero.

Format is:

• optionally natural number representing total amount of generic mana (use key *)
• optionally followed by any combination of w, u, b, r, g (case insensitive in input, return lower case in output), each representing one mana of specific color.

If case of Strings not following specified format, return nil/null/None.