Browser bundle is very large

I'm currently trying to use this library for isomporphic form validation/rendering. However, this lifted my (uncompressed) JS size from ~700kb to 3.1mb.

After removing `formidable`, `async` and `qs` as dependencies (formidable has the largest contribution), and leaving the server implementation to parse form data, my bundle size is sitting at around 900kb. [You can see the fork here](https://github.com/hackcambridge/forms)

You could also remove the dependencies on shims (`object-keys`, `is`, `object.assign`, `string.prototype.trim`), if you just specified that people who want support for older browsers/runtimes need to include polyfills themselves.

That will result in zero dependencies with almost no loss in functionality. The changes being:
- You have to parse the form data before handing it over
- You have to polyfill JS features that aren't available

I know legacy support is an important feature for this library, as is ease of "plug and play" use, so I wanted to see if there was any interest in reducing dependencies.