Expose feature detection.

[lgarron]

See issues #23 and #25. Where possible, it would be nice to detect ahead of time which of the following will work, and expose an API to query it. In response, a developer could e.g. use a Flash fallback only when necessary.

• Plain text copying
• Rich text copying
• Pasting

Since a successful copy/paste has a user-visible side effect, it would be nice to avoid triggering them unless you actually want to perform the action.

[lgarron]

It seems document.queryCommandSupported("copy") will get us most of the way there.

[lgarron]

There isn't a great way to do this on older browsers, where copying is best-effort.

For new browsers, this is now covered by the following issue in the spec: w3c/clipboard-apis#67

I'd prefer not to add feature detection that should go in the API itself.

[lgarron]

Closing in favor of the spec issue.