[gitbook] Copy to clipboard buttons for code blocks

[RLesur]

The aim of this PR is to resolve #775.

I propose the following implementation:

• use clipboard.js: this is a maintained lightweight JS library for copying to clipboard. It is already used by pkgdown. There exists a gitbook copy-code plugin but it seems to be unmaintained. @yihui, tell me if you agree to integrate clipboard.js in bookdown or if you would prefer to use the copy-code plugin.

• a very basic styling for copy-to-clipboard buttons. @behrman, contrary to pkgdown there is no tooltip. For tooltips, we should add another library in bookdown and I feel that the extra benefit of tooltips would be very small.

[behrman]

I've tested the feature branch, and it seems to work fine. We do get a tooltip if we hover over the copy-to-clipboard icon for a sufficient amount of time. I think this is fine.

[RLesur]

@behrman For the tooltip, we do get the user agent button tooltip. Its design and styling may vary from one browser to another.

The main difference with pkgdown is that the tooltip content is not updated after a succesfull copying (the user sees Copied! with pkgdown).

There is another difference: in case of failure, there is a specific message in the pkdown tooltip, see https://github.com/r-lib/pkgdown/blob/d01672efa3baa80fd7ab238b7143080b0daf49a4/inst/assets/pkgdown.js#L108-L110
With the current PR, the copying action can silently fail.

[behrman]

@RLesur You've done a fine job with the design of the tooltip messages for pkgdown. There may be some advantage to have consistent behavior between that and bookdown, though I don't know the downsides of doing this. I think the current behavior is fine.

[yihui]

Looks perfect. Thank you!