Server side capability via jsdom

[bceskavich]

While working with convert-rich-text at Vox Media, we've come across the need to use this package on both the server and the client. Currently, this only works on the client as it requires access to the DOM. This PR includes changes to add this functionality:

• Two package entries, one for the browser and one for the server. The server entry instantiates a global document variable that is a jsdom instance
• add functions in formats objects can include an optional third argument called dom. If used, we'll pass in our own helper, instead of asking the user to provide their own.

[thomsbg]

Is there a way to reduce duplication between browser.js and index.js? Perhaps the convert function could take in document as an option?

// index.js
var document = require('jsdom').jsdom();
module.exports = require('./browser')({ document: jsdom() })

[thomsbg]

Thanks @bceskavich!