Add support for multiple top-level instances in rbxmx and rbxm formats

[LPGhatguy]

The Roblox model formats already support having multiple instances at the top-level. This is how places are encoded!

Rojo right now will fail to load a model file that has more than one top-level instance. The rationale was:

• It's a bit of extra complexity to make Rojo support returning multiple instances from a single file
• It's unclear what names should be assigned to instances if there are multiple of them -- should Rojo switch to using the names of the instances instead of the file name?
• Not many users know that models can contain multiple top-level instances

Stuff we should answer:

• How should we name instances?
  • [ ] Never use the file name
  • [ ] Use the file name only if there's one top-level instance
• Does this feature impact two-way syncing?