Add better upgradeability

Currently one must override the already installed binary via an explicit version install with the -f force flag: 

```
deno install --allow-read --allow-write --allow-net --allow-run --allow-env -f --name codeup https://deno.land/x/codeup@2.0.0-beta.4/main.ts
```

This is not great for cli-tool upgrades, ala `deno upgrade`

=> Therefore: build upgrade command into the cli tool itself