script hash rails integration PoC

Application demonstrating this: https://github.com/oreoshake/script_hash_test
Video demo: https://www.youtube.com/embed/Bc2hvziTRxg

Two pieces:
* Rake task to find all inline script, compute the script-hash value, store in config/script_hash.yml

AND 

* Contains new before_filter to subscribe to ActiveSupport::Notifications events (rendering partials/templates)
* All rendered templates write to an env value that contains all hashes for each rendered template/partial
* Middleware then (stupidly) does a find/replace or adds a directive containing the script hash values

Based on "simple" script hash proposal.
Need to verify hashed values with various methods.

This makes me think the before_filters should create a ContentSecurityPolicy object and build the entire header in the middleware.
This causes improperly formatted `script-src` when there's multiple script hashes.  As is, the output for two hashes, `abcde` and `fjhij` is:

    script-src 'sha256-abcde fghij'

This should be:

    script-src 'sha256-abcde' 'sha256-fghij'

Here's the proper source code:

```ruby
def hash_source_expression(hashes, format = "sha256", delimeter = "-", hash_delimeter = " ", wrapper = "'")
  hashes.map { |hash| wrapper + format + delimeter + hash + wrapper }.join(hash_delimeter)
end
```

If you like I can make a PR to this branch that fixes.