Add fallback theme when no ThemeProviders on top

[js-jegan]

Context

• There are cases when there is no ThemeProviders on top, in that case we should enable a fallback mechanism where it defaults to a theme.
• This feature enables the styled-theming to be flexi-independent of any libraries like ThemeProvider
• Please refer to the examples in the Readme for the same.
• Unit tests(in progress...)

Example

import styled, { ThemeProvider } from 'styled-components';
import theme from 'styled-theming';
const backgroundColor = theme('mode', { light: 'lightgray', dark: 'darkgray' }, 'light');
const Button = styled.button`
  background-color: ${backgroundColor};
`;
// Example #1 - bg color of this will be lightgray because no ThemeProvider as it's ancestors
<Button /> 
// Example #2 - bg color of this will be darkgray because it has ThemeProvider with a valid theme prop
<ThemeProvider theme={{ mode:'dark' }}>
  <Button />
</ThemeProvider>

[js-jegan]

cc @jamiebuilds