CORS implementation

## Implements CORS middleware/DSL.

On `Hyperdrive::DSL::Main` `CORS` options are passed in thorough `#cors` as a hash.
Keys are sanitized and set to a hash instance variable `@config[:cors]`
`@config` provides one place for all configurations to live on the `Hyperdrive::DSL::Main` level.

On `Hyperdrive::Middleware::CORS` default CORS options are provided. Default values were based on https://github.com/britg/sinatra-cross_origin.
`#format_options` takes in options from the DSL level and formats keys and values to be valid CORS headers.