Implement ROLE filter

Target roles by including an env variable:

    ROLES=app,web cap production deploy

Or by setting `filter` or `select` variables:

    set :filter, roles: %w{app db}

Both variables will be applied if both are defined

Closes #446, depends on https://github.com/leehambley/sshkit/pull/29
Merge pull request #621 from capistrano/feature/role-filtering

Implement ROLE filter