Add dredd_command option

[gonzalo-bulnes]

As a developer
In order to be able to use Dredd::Rack with installations of Dredd that are not global
And, for example, use it inside a Docker container
I want to define a custom Dredd command
And I want that configuration to be used by all the Rake tasks I define

See #25

[gonzalo-bulnes]

Does this look good to you @gottfrois?

[gottfrois]

It seems to do the job but why not a runner configuration directly? Since it's, in the end, a runner instance variable.

[gonzalo-bulnes]

I thought that the setting would be shared by all the runner instances within a project (much like the application definition), and that it would be convenient to avoid unnecessary repetition accross all the Rake tasks, for example. What do you think?

[gottfrois]

Yeah that makes sense.