pip_install plugin

It installs Python packages from pypi into the bootstrapped system.

Configuration looks like

```json
{
//...
    "plugins": {
        "pip_install": {"packages": ["a_package", "another_package==1.0.1"]}
    }
}
```
It installs `build-essential` and `python-dev`, so Python extension modules can be built.

See discussion in #103 and docs in #105