Support for top-level user configuration (#1899)

* Added support for top-level user configuration

At the user level, a TOML config can be specified in the following locations:
* Windows: ~\.black
* Unix-like: $XDG_CONFIG_HOME/black (~/.config/black fallback)

Instead of changing env vars for the entire black-primer process, they
are now changed only for the black subprocess, using a tmpdir.