Dedupe wurd-web and wurd-node

[caub]

There seem to be similar stuff between the 2 libs

https://github.com/wurdcms/wurd-node/blob/master/src/block.js#L166

https://github.com/wurdcms/wurd-web/blob/master/src/block.js#L105

I think we could merge them and make sure it works on both node and web (for example we can require('node-fetch') on both node and browser, node-fetch is 'universal' (https://github.com/node-fetch/node-fetch/blob/master/package.json#L6)