Add support for imported views

[asg017]

I haven't tested this, but I'm pretty sure this won't work:

Notebook 1

viewof a = DOM.input()

md`Typed value: "${a}"`

Notebook 2

import {viewof a} from 'notebook1'

viewof a

md`This notebook's value: ${a}`

Supporting this viewof import would be awesome!

[bryangingechen]

I can tackle this and #2 if you're not already working on this.

[asg017]

@bryangingechen sure, that would be awesome!

The parser already tells us if the imported cell is a view or not, so this issue should be a simple if statement on that boolean, then importing the viewof x instead of x. Implemenring the mutable keyword may be a little more fun...