Support Sublime path placeholders in configuration

[danpalmer]

Sublime Text provides some placeholders such as project_path that can be used in configuration.

To configure Sublack currently, this must be hard-coded:

{ "black_command": "/path/to/my/venv/bin/black" }

As each project will typically have its own virtualenv, this configuration needs to be repeated in every sublime-project file.

By supporting the placeholders, the following can be done instead:

{ "black_command": "${project_path}/.venv/bin/black" }

Now any project with a .venv will work without further configuration.

These placeholders are supported by many plugins, including most of the SublimeLinter plugins, so this pattern works for flake8, isort, mypy, pylint, and others. None of these require extra project-specific configuration to make them work because of this. It would be great to get this support in Sublack for black.