[material-ui-icons] add making index.js

[taichi]

material-ui-icons module suppports default member imports only.

this PR contains supporting member name imports.

for example,

• now

import AddIcon from 'material-ui-icons/Add';
import AllOutIcon from 'material-ui-icons/AllOut';

• this PR supports

import { Add, AllOut } from 'material-ui-icons';

[mbrookes]

Well, we had https://github.com/callemall/material-ui/blob/master/scripts/icon-index-generator.js, but I left it out on next because adding one icon that way adds them all (without tree shaking).

[oliviertassinari]

without tree shaking

@mbrookes I think that we should let people taking advantage of the tree shaking.

[oliviertassinari]

Actually, I think that we should update the documentation on that matter.

[mbrookes]

@taichi Please could you update https://github.com/callemall/material-ui/blob/next/packages/material-ui-icons/src/README.md which is included in the NPM package?

[oliviertassinari]

Thanks, I'm gonna tweak 2-3 things.