Enable/Allow to zoom in/out

[pombredanne]

Since the WB is backed by a web browser, I would expect to be able to zoom in and out (Ctrl+ and Ctrl- typically) and this is not enabled. It would be great if it were

[vsvipul]

@pombredanne i have opened a pr. please review.

[steven-esser]

@vsvipul See my PR review here: #357 (review)