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 !
1. Need to get rid of the passive voice here. Recast to something like: You may sometimes need to expose devices directly to a container.

2. Need to have single back-ticks (`) for code style in markdown. (There are multiple instances of this throughout, I'm only flagging this one.) Also, please add "The" to avoid starting a sentence with lower-case.