Add command for adding function models

[lordidiot]

Manticore has models for common library functions. These help to improve performance, as opposed to symbolically executing the instructions of the original function.

Adding a command to apply these models would be helpful to expose these features to users less familiar with manticore.

Additionally, we could leverage on the Binary Ninja analysis API to try to apply these function models automatically (perhaps with a warning/prompt for the user).

Documentation for usage of function models: https://github.com/trailofbits/manticore/blob/master/docs/native.rst#function-models