Command-line option to specify resolver

[mlafeldt]

At the moment, the resolver that is used to set up cookbook dependencies is automatically selected based on the files in the current directory. There are cases where more than one resolver could be used, e.g. if there's a Berksfile and a Cheffile. Provide a command-line option to specify which resolver to use. For example like this:

$ chef-runner --resolver berkshelf
$ chef-runner --resolver librarian
$ chef-runner --resolver dir