adding npm scripts for compiling ES5 dist through babel

[criography]

This PR adds a build script that would allow to use this library in opinionated environments ESNext is not fully supported, like create-react-app.

It doesn't alter the source at all.

[onury]

Thanks @criography. I'd like to merge this but 2 points:

• Should better make it universal module at once (via webpack maybe) so it'd work directly in the browser too.
• We might still have a /dist dir but import would be prettier like:
  import { invert } from 'invert-color'

Would you mind updating your PR for this?

[criography]

1. It was simpler to just use babel's plugin to achieve UMD.
2. I wasn't sure if this is what you meant, but if the path is not specified, both require and import methods will use whatever is set as main declaration in package.json. At least that's my understanding - I don't have much experience in this field.
   In this commit, I set main to dist output, which seems to be the common strategy for libraries out there. That said, I'm open to suggestions how to handle it differently.

[onury]

invert object is not exposed properly in DOM. It was exported as window.index per the filename.

[onury]

This way in DOM, it's exposed as window.index, not window.invert.

[onury]

It was simpler to just use babel's plugin to achieve UMD.

That's better.