Consider handling nested configuration

[amyreese]

@ofek in #186 uncovered potential confusion in repos with nested pyproject.toml files, where black appears to also look at the parent project's configuration, while µsort only looks at the nearest configuration. We may need to look at how black handles this, and either use black's API for loading the line length, or extending our configuration loading process to continue searching if no [tool.usort] or [tool.black] sections are found.