Calc plugin should copy to clipboard when ENTER is pressed

[jaysondc]

A common use for a calculator is taking the number and copying it into some form or document. If I use dext as a calculator this isn't possible as clicking away from dext erases the answer.

[vutran]

We should add a new plugin action that allows copy to clipboard. I believe @adnasa made a PR in the past for this here: #39.

It may be worth it to re-open that since I can see this being useful for dext-emoji-plugin as well since.

[adnasa]

I recall that we discussed that we should copy through mods and not through a specific action defined by the plugin

[vutran]

mod was for alternative title/subtitle/args.

We went with the Alfred flow if cmd + c instead of enter but since some of our plugins doesn't necessarily require an action of openurl or exec, it may make sense to expose the implementation you did previously as well. I think this will provide a better user experience since some people may just hit enter and assume it did something.