Build es6 module that can be used with an import statement

[m0ose]

I am working on a sketch that requires a version of tesseract that works with an import statement. In order to avoid a build step it would be nice to have a normal es6 module.
This way it can be imported from the browser like so ...

import Tesseract from '../node_modules/tesseract.js/dist/tesseract.es6.min.js'

I put an extra build option in the package.json file which uses rollup to make an es6 module.

[jeromewu]

Sorry for pending this PR for so long.
But for me it would be much better to use webpack to build the es6 version instead of using rollup.

[Balearica]

While it would be ideal to continue using a single build system, it looks like Webpack does not currently support this. (There is an experimental feature, however I wasn't able to get it working, and looking at the relevant Git Issue many others are having problems as well.)
webpack/webpack#2933

Therefore, as I think there is significant benefit to offering an ESM module in the current year, I will merge this version in after confirming it still works and adding the necessary tests.

[Balearica]

Merged a modified version of this code in using a different PR (#659). Thank you for contributing.