Added url list type

Added ``url`` list (and show) type for easier creating external urls and links to other parts of the project.
This can be solved with custom template but will be nice if we will have some default solution for such common problems.

Related SonataDoctrineOrmAdmin PR: sonata-project/SonataDoctrineORMAdminBundle#291

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -