webview tag doesn't work

[alexchandel]

This is similar to #75. webview is a custom tag used in Chrome Packaged Apps. Apparently Google likes to use custom tags, so there should be a CoffeeCup means to generate arbitrary tags from certain unknown function.

There should be a builtin way to register unrecognized symbols as tags in CoffeeCup, either with C-preprocessor syntax or a builtin function:

##register(webview)
div ->
    webview ->

coffeecup(tag: 'webview')
div ->
    webview ->

[10xLaCroixDrinker]

Custom tags are supported