Add option for templates (for automatic package installation)

This is related to https://github.com/np-8/venvlink/issues/4.

### Proposal

Sometimes it would be handy to install some packages automatically, but putting all useful packages to `autoinstall_packages` does not make sense. Something like "virtual environment templates" could be useful. In short, add to `venvlinkrc` something like:

```
[templates]
datascience = [numpy, pandas, matplotlib]
datascience2 = [numpy, pandas, plotly]
django = [django~=3.1.5, django-debug-toolbar, djangorestframework]
```
There could be optional PEP440 compatible version identifiers (like in the django case in the example above) to help keeping versions compatible.

Usage could looks something like this. Creating new virtual environment:

```
python -m venvlink --template datascience
```

or

```
python -m venvlink -t datascience
```