Support user/local configs

[calebcartwright]

Description

Allow for the hook configuration for a project to be augmented/overriden by a local/user configuration

Value

This would provide greater flexibility for individuals when working on a project. A project-level default could be set (using the existing .rusty-hook.toml configuration file), but then users could individually have their own configuration file (perhaps .rusty-hook.local.toml or something like that) if they wanted to run additional/different/etc. hooks in their local environment.