Auto select and/or copy shell command examples on click

[leobalter]

This is based on the results from the google analytics and heatmaps analysis.

On the getting-started page there's a lot of clicks on the code examples, specially on the shell command examples as npm install -g grunt-cli.

We could make life easier for those developers and see if a copy tool would help them. (at least auto selecting the entire text).

[XhmikosR]

Shouldn't be too hard with clipboard.js.