No way to fetch / serve from different URLs

Right now the `process.env.EMBER_CLI_INJECT_LIVE_RELOAD_BASEURL` is used in the script tag and the server, which gets kind of hairy when supporting multiple environments.

It would be really convenient for us to be able to override `liveReloadBaseUrl` with something like this:
```
{
"liveReloadServerBaseUrl": "/",
"liveReloadScriptBaseUrl": "/ember-app/"
}
```

Let me know what you think about this. Glad to open a PR.