Add command to print the current value of the inspector.

[r0man]

This PR adds the interactive cider-inspector-print-current-value command that prints the current value of the inspector to the cider-inspector-value-buffer with the usual pretty printing machinery.

The NREPL part is here: clojure-emacs/cider-nrepl#933

When pressing "P" in the inspector the currently inspected value is printed in a new popup buffer (on the left side in the picture).

I see it as the inspectors brother of cider-pprint-eval-last-sexp which I use all the time.

This is the functionality I initially suggested, before working on the pretty mode. I think it is still useful. What I would like to have is a quick way to pretty print the current value of the inspector in the same familiar way my eye is used to, as the Clojure pretty printer (or what we are using in CIDER).

The new pretty mode does print more pretty, but differently, it uses "inspector mode" printing.

That's why I think this could be a useful addition.

Wdyt?

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