Command line switch for configuration overlay

To specify a Clojure version, I can now use the following `$PWD/deps.edn`:

```
{:deps {org.clojure/clojure {:mvn/version "1.9.0-RC2"}}}
```

To simplify automation it would be useful to configure this setting using a command line argument, by supplying a file

```
clojure -Fadditonal.config.edn
```

or a string

```
clojure -G'{:deps {org.clojure/clojure {:mvn/version "1.9.0-RC2"}}}'
```

These switches could either replace `$PWD/deps.edn`, or overlay (and possibly override) it.