Fluid interface - proof of concept

Hi,

Nice project! :+1: 

I just thought that would be nice to have a more fluent interface to Is. For instance:

    if (is(x).equal(y)) {...}

I'm sending a proof of concept implementation to your appreciation. The only problem with the current code is that now it has state (predefinedValue) and can cause errors on sequencial calls without this value.

Anyway, I thought it would be nice to have this on Is :)