Setup Dependabot for automatic dependency updates?

Currently, `crossbeam` could be updated:
```
strohel@mat480s ~/projekty/actor $ cargo outdated -R
Name       Project  Compat  Latest  Kind    Platform
----       -------  ------  ------  ----    --------
crossbeam  0.7.3    ---     0.8.0   Normal  ---
```
...but I'm deliberately not updating it, so that the the automated way could be tested right away.