Monoprice Select Mini extended bed brackets

Extended the bed brackets to utilize the 300x200mm Anodized Aluminum Y Carriage Plate XL Upgrade for Prusa i3.

I have not tested, printed or fit as I have yet or order the carriage plate. If successful it will use the carriage, heat bed and borosilicate glass from the prusa I3 for the monoprice select mini.

6/19/17: Changed a few things on the front rail after installation.

Remixed bed rail mounts from https://www.thingiverse.com/thing:1916033.

6/25/17: I added the wider bed rail m