Add option for automatic installation of packages

This was briefly mentioned in https://github.com/np-8/venvlink/issues/3.

### Proposal

It would be handy to have an option for automatic installation of packages in `venvlinkrc`, like `install_packages` or `autoinstall_packages`. This could be a list, like `[numpy, matplotlib]`.

Versions could be defined as defined in [PEP440](https://www.python.org/dev/peps/pep-0440/); `numpy~=1.19.5` for numpy versions `1.19.x` but greater or equal than `1.19.5`.

Example (venvlinkrc):

```
autoinstall_packages = [numpy~=1.19.5, matplotlib]
```