"command" in onCustomCommand

@rmevans9 is already working on this.

Since May 17, 2019.

Right now when you set up a custom command, you need to supply it with a mandatory handler and a "command" property; eg.:

```
Reactotron.onCustomCommand({
command: 'command 1',
handler: () => console.tron.log('hi')
});
```

Unfortunately, this property, while mandatory, seems to be totally useless. The optional "title" property actually seems to serve whatever functionality this "command" property should have served:

```
Reactotron.onCustomCommand({
command: 'command 1', // Do nothing but is mandatory
title: 'command 1', // Actually sets the command's name in the Reactotron UI
handler: () => console.tron.log('hi')
});
```

I looked into the codebase and it's only being used to ensure uniqueness (https://github.com/infinitered/reactotron-core-client/blob/06648fb8b7fb4cfe5eec37f81ba3e974b8a44318/src/reactotron-core-client.ts#L441) but I don't really see the point of guarding against that; even if we do want to do that, that can also be easily achieved using either the ID or the "title" property.

I suggest we merge these two properties into one, potentially still calling it "command" to avoid a breaking change.