dev-cmd/contributions: Switch `--email` to be a named arg
dev-cmd/contributions: Switch `--email` to be a named arg

- This way we can do the more intuitive:

```
$ brew contributions --repos=brew,core rslpolster@gmail.com
Person rslpolster@gmail.com directly authored 1580 commits and co-authored 125 commits to brew, core in all time.
```

Homebrew/brew,Homebrew/brew,JCount/brew,nandub/brew,Homebrew/brew,Homebrew/brew,nandub/brew,MikeMcQuaid/brew,JCount/brew,JCount/brew,MikeMcQuaid/brew,nandub/brew,JCount/brew,nandub/brew,MikeMcQuaid/brew,MikeMcQuaid/brew