Make delete a top level command

Most boom commands are passed as the first argument, with the list or item coming after:
```sh
boom urls github "https://github.com"
boom echo github
boom random urls
boom open github
```

Until recently, delete was an odd duck - it was passed _after_ the list/item:
```sh
boom urls github delete #boo
```

As part of the 0.3.0 release, the syntax for deleting an item was changed to be less ambiguous:
```sh
boom github --delete #hmm
```

I think putting delete first, like all of the other commands, makes things even clearer:
```sh
boom delete urls github #yay
```

This is something I was considering a while ago, but I didn't want to break existing behaviour. Since 0.3.0 already did that, now is probably the best time to think about making this change.