add support for disabling vcr_cable with an environment variable

should resolve #3
I added support for parsing erb in the yml so that I could read in my environment variable.
When I run this in my app my config file looks like this:
```
development:
  hook_into: webmock
  cassette_library_dir: development_cassettes
  allow_http_connections_when_no_cassette: true
  disable_vcr_cable: <%= ENV['DISABLE_VCR_CABLE'] %>
```