Add method for @js macro with base (à la Blink)

[twavv]

Blink defines a method for @js that looks like

@js window console.log("foo")

We should define a base method for here it in this repo and then let other packages (e.g., Blink and WebIO) extend a method that acts on the specific type of the first arg.

# In JSExpr
macro js(target, expr)
    return :(js($(esc(target), ...))
end

# In Blink
function JSExpr.js(w::Window, code::JSString)
    # Evaluate the JS code in the Blink window
end