Ready to merge? Another attempt at script hash integration

It's been a loooong time coming and things have gone back and forth, but I think this is ready for use. See https://github.com/oreoshake/script_hash_test for an example. The following script/style tags are whitelisted by nonce AND hashes.

```erb
<!-- Hashes -->
<script>
console.log("Hashed manually")
console.log("")
</script>

<%= hashed_javascript_tag(raise_error_on_unrecognized_hash = true) do %>
console.log("hashed automatically!")
<% end %>

<!-- script nonce -->
<script nonce="<%= @content_security_policy_nonce %>">
console.log("manually nonced")
</script>

<%= nonced_javascript_tag do %>
console.log("nonced!")
<% end %>

<%= nonced_javascript_tag 'console.log("nonced without a block!")' %>

<%= ENV["RAILS_ENV"] %>
<%= @content_security_policy_nonce %>

<!-- style nonce -->
<style nonce="<%= @content_security_policy_nonce%>">
body {
  text-decoration: bold;
}
</style>
<%= nonced_style_tag 'text-decoration: underline;' %>
<%= nonced_style_tag do %>
  text-decoration: lol;
<% end %>
```

`hashed_javascript_tag` computes hashes dynamically ONLY IN TEST AND DEVELOPMENT ENVS. You can configure this to raise an error or print out to the console when an unrecognized hash is computed. This allows you to develop and not worry about the actual hash values until the code is stable. In production, hashes are NOT computed dynamically and things will break if the rake task to generate the values is not run.

High level description:
* A rake task that does a simple search for inline script - either a script tag without a src or a block with the provided view helper (hashed_javascript_tag)
* The hashes are stored in a yaml file with the filename as the key, and the hashes as an array of strings
* When the script hash middleware is in use, the config value is stored in env.
* All files that are rendered that correspond to an entry in the script hashes config file, the hashes for that file are saved to env
* When the middleware runs, it pulls the config and hashes from env and sets a header

Changes:
* Adds view helpers for nonced and hashed script tags (and nonced style tags)
* Add middleware for supporting the automatic hash inclusion