add support for prop value in mapToTheme

[mmartinsky]

Closes #17

Allows users to add an optional second argument to mapToTheme, as below:

import styledMap, { mapToTheme as theme } from 'styled-map';

const myTheme = {
  buttonColor: {
    primary: 'orange',
    warning: 'red',
    info: 'blue',
    default: 'white',
  },
  ...
};

const Button = styled.button`
  color: ${theme('buttonColor', 'kind')};
`;

<Button theme={myTheme} kind='warning'>Click</Button> // will be red

[scf4]

Looks good — I'll review it soon, thanks!