CR-10 pen holder

This is a remix of the [Hypercube pen holder](https://www.thingiverse.com/thing:2459624) to fit onto the CR-10 (although not as fancy as his [updated version](https://www.youtube.com/watch?v=HciNR-tskwQ)). This pen holder can be used for pens with a diameter between 8 and 11 mm.

NOTE: this pen holder can only be used if the fan mount has been removed (and for example replaced by an [OEM Fang](https://www.thingiverse.com/thing:2390346)), so the two remaining holes can be used.

Furthermore,