Make js-ipfs-api modular

[daviddias]

Users would be able to achieve really small js-ipfs-api bundle sizes if it was structured like async or lodash, where a user can pick only a subset of the module to be bundle into their app.

Example with async:

const parallel = require('async/parallel')

We want to achieve this for js-ipfs-api, so that users don't have to create their own sharded forks (ie. https://github.com/SilentCicero/ipfs-mini)

Ideally, this should work so that both const files = require('js-ipfs-api/files') and const add = require('js-ipfs-api/files/add') should work.