Fix UMD implementation

[doughsay]

The current UMD header and footer prevent ansi_up from being used by webpack. The kind folks over at StackOverflow helped me out with my question. I used these UMD templates to come up with something that seems to work much better.

This will:

• export the AnsiUp constructor to the browser as a gloabal.
• allow webpack+babel or babel on its own to import it like this: import AnsiUp from 'ansi_up'
• keep the same node behavior as before: const AnsiUp = require('ansi_up').default

[doughsay]

Another thing to consider: technically we can make the node use-case easier too by changing the CommonJS section to:

var exp = {};
factory(exp);
module.exports = exp.default;

This would allow node usage to be const AnsiUp = require('ansi_up') without the need for .default

[drudru]

Hi - thanks - will review and push/etc tomorrow.

[drudru]

Thanks!