Ability to create rich gui inside of Notebook

Discussed on mailing list http://mail-archives.apache.org/mod_mbox/incubator-zeppelin-dev/201503.mbox/browser

Basic idea is adding '%angular' to display system.
Like currently supported '%html' that prints output as html code, '%angular' displays output as angular view. For example,

println("""%angular Your name is {{ z.name }}. <a ng-click="z.onClickOkay()">ok</a>""")

will compiled and rendered as angular view code. For variable (z.name) and function (z.onClickOkay()), we can develop some kind of Js(angular controller) - JVM gateway so user will able to bind Object or Function into the angular scope, like

val name = "Zeppelin"
z.angularBind("name", name)

def onClickOkay() = {
    ...
}
z.angularBind("onClickOkay", onClickOkay)

In this way, user can implement their own customized visualizations and user interface without (almost) any limitation.