[Typescript] createPalette and createMuiTheme should accept a DeepPartial<Palette> instead of Partial<Palette>

[xaviergonz]

Right now createPalette and createMuiTheme accept a Partial, but actually they should accept a DeepPartial since they do a deepmerge instead of a shallow merge

type DeepPartial<T> = {
  [ P in keyof T ]?: DeepPartial<T[ P ]>;
};

• I have searched the issues of this repository and believe that this is not a duplicate.

Expected Behavior

Using a palette like this should not give type errors:

createPalette({
  shades: {
    dark: {
      text: {
        primary: 'green',
      }
    }
  }
});

Current Behavior

It gives a compilation error, forcing you to cast it to any or to fill in the complete sub-types.

Steps to Reproduce (for bugs)

See code above.

Your Environment

Tech	Version
Material-UI	latest beta
React	16.2.0
browser	Chrome 62
TS	2.6.2