Feature: ability to add custom accessors

[todofixthis]

As a developer, I would like to define custom accessors, so that I can work with env vars in formats specific to my application's requirements.

For example:

const env = from(process.env, {
  asNodes: (raiseError, value, type = Set) => {
    return new type(
      value.split(',')
        .map(s => s.trim())
        .map(s => s.split(':', 2))
        .map(([ip, port = 80]) => ({ ip, port }))
    )
  }
})

const bootnodes = env.get('BOOTNODES').asNodes()

Proposed acceptance criteria:

• A developer can call from() and provide an optional object as a second parameter. The object defines additional accessors for any value returned by that instance's get().
• If the name matches an existing accessor, the existing accessor is replaced by the new one for all subsequent calls to env.get().

[evanshortiss]

@todofixthis I like this, there's a guide in the README for making a PR adding custom accessors, but sometimes they are application specific so this is great 👍

Will take a look at the PR

[todofixthis]

Per #72 (comment) modifying the acceptance criteria so that extra accessors are provided as a parameter to from() rather than using a method to mutate the object state.