Add enter handler for the textbox

[VijithaEkanayake-zz]

Fixes: https://github.com/h2oai/wave/issues/372

[mturoci]

@mturoci - this might conflict with #348?

Yes, I suppose. I suggest addressing both issues in this PR.

[VijithaEkanayake-zz]

The implementation unconditionally calls the keyup handler. We need a properly-named attribute on the textbox that conditionally enables this behavior.

@mturoci - this might conflict with #348?

@lo5 @mturoci Introduced a new attribute called submit to the textbox to conditionally call the keyUp handler and addressed the requested changes.