Re-implement `esbonio.sphinx.configOverrides`

[alcarney]

The brings back the sphinx.configOverrides option from the 0.x version of esbonio, offering a convenient way to override parts of a project's conf.py file

{
    "esbonio": {
        "sphinx": {
            "pythonCommand": ["/path/to/venv/bin/python"],
            "buildCommand": ["sphinx-build", "-M", "dirhtml", "docs/", "docs/_build"],
            "configOverrides": {
                "html_theme": "alabaster",
                "html_theme_options": {},
            }
        }
    }
}

Closes #785