Add --device flag to allow additional host devices in container

I know I said I was out for the weekend, but I got up **early** this morning, and with a bit of repasting(it's like rebasing, but for the unenlightened), I bring you:

We add a --device flag which can be used like:

````
$ docker run --device /dev/sda:/dev/xvda:rwm ubuntu /bin/bash
````

To allow the container to have read write permissions to access the host's /dev/sda via a node named /dev/xvda in the container.

Docker-DCO-1.1-Signed-off-by: Timothy <timothyhobbs@seznam.cz> (github: https://github.com/timthelion)

Big thanks to @dineshs-altiscale !
Change beginning to: The `--device` flag

Replace exclamation point with a period.