No way to fetch / serve from different URLs

[c-brooks]

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.