Handle "container" option of clipboardJS

[enna]

This PR handles the container option of ClipboardJS, as described here.This container option allows the clipboard to work in modals, which will fix issue #180

[jkusa]

@enna thanks for picking up #180! A few comments:

• Can we call the arg container (in alignment with Clipboard.JS) and do something like this:

  container : typeof container === 'string' ? document.querySelector(container) : container
});

• Also, would you mind documenting this option in the README?

[enna]

@jkusa sure, no prob. :)

[jkusa]

Suggested change

	- `container` - selector string of element, or element, containing the component. If you want to use the component inside a modal, this property is mandatory and should referred to the modal element.
	- `container` - selector string or element object of containing element. "For use in Bootstrap Modals or with any other library that changes the focus you'll want to set the focused element as the container value".

[jkusa]

@enna change look good. Just a small tweak to the readme and we should be good.

[enna]

All should be good now @jkusa :)

[drewdaemon]

We could really use this improvement! Any plan on cutting an NPM release with container support @jkusa?