Adds support for Erb directives in newrelic_plugin.yml

This makes it possible to pull configuration settings in from the environment. For example, one could configure the plugin automatically in a Heroku app like this:

```erb
<%
  require 'uri'
  uri = URI.parse(ENV['DATABASE_URL'])
%>

newrelic:
  license_key: <%= ENV['NEW_RELIC_LICENSE_KEY'] %>
  # verbose: 1

agents:
  postgres:
    host: <%= uri.host %>
    port: <%= uri.port %>
    user: <%= uri.user %>
    password: <%= uri.password %>
    dbname: <%= uri.path.split('/')[1] %>
    sslmode: 'require'
```