adding support for port ranges on --expose

Addresses #1834

Signed-off-by: Srini Brahmaroutu <srbrahma@us.ibm.com>
I know you didn't make this wording, but maybe we can change this now. ping @SvenDowideit 
I think this whole paragraph is not right.  Technically `expose` doesn't make any ports available inside the container.

```Markdown
advertises ports which the process(es) **in** the container is listening on for incoming connections for the purposes of
service discovery.
In cases where **inter-container communication** is disabled, this will allow **linked**containers to communicate
over these ports and these ports only.
```
Or something like that.