Various fixes

This accumulates mainly bug fixes and some minor improvements made over the past while that have been overdue for a
release.

The biggest change is that the library defaults to Jason for json processing now.

Add async recaptcha

Features

• Invisible recaptcha working
• Async loading of grecaptcha javascript file

Misc

• Adds mix format

Update dependencies

This release includes an update of dependencies and other minor changes.

Allow runtime environment config

Configuration can now accept any of the following:

• Standard variables:

config :recaptcha,
  public_key: "some public key string",
  secret: "some private string"

• Environment variable at compile time:

config :recaptcha,
  public_key: System.get_env("RECAPTCHA_PUBLIC_KEY"),
  secret: System.get_env("RECAPTCHA_PRIVATE_KEY")

• Environment variable at runtime:

config :recaptcha,
  public_key: {:system, "RECPATCHA_PUBLIC_KEY"},
  secret: {:system, "RECAPTCHA_PRIVATE_KEY"}

Thanks to @kennyballou for the addition 👍

Support poison 1.5

Support for Poison 1.5 added to the mix dependency list.

Version 2.0 rewrite

A rewritten API. See the README and code documentation for details.