Allow screenfull to work in non-browser JS environments

[gfx]

I'd like to this library for my project, which works both on browsers and nodejs. Of course, the fullscreen feature is only meaningful for browsers but some React components, which might include this library, are used for server-side rendering.

This PR just checks window and lets enabled return falsey and it's enough for react components. Now nodejs can load this library with the enabled property:

$ node -e 'console.log(require("./src/screenfull").enabled)'
undefined