Add Rules support

This request would depend on:
#1458418: Add masquerade events API

And therefore, this request had already been moved to (somehow I forgot about this post):
#1458334: Add Rules support

Then, for example, the implementation of capturing when a user is about to masquerade would be:

if (module_exists('rules')) {
  // rules_invoke_event(<event_id>, <current_user>, <new_user>)
  rules_invoke_event('masquerade_X', $user, $account);
}

I thought initially about going down this road, but it feels too tightly coupled.

Plus, in further thought, we could let other modules stop the masquerade based on custom requirements.

$roll_with_it = TRUE;
if (module_exists('rules')) {
  // rules_invoke_event(<event_id>, <current_user>, <new_user>, <permit_masquerade>)
  rules_invoke_event('masquerade_X', $user, $account, $roll_with_it);
}
if (!$roll_with_it) {
  // Do something to let the user know the masquerade wasn't permitted.
}

@citricguy
That's not quite the case.
The Masquerade module needs to fire the basic hooks necessary for rules to work. Otherwise, the rest of this is not possible.
(For example: module_invoke("masquerade_start")). This thread already suggests the basics for what would be required. #1547058: Add hooks when user starts or stops masquerading

My original suggestion was wrong because rules integration would be coupled to masquerade with hooks specific to rules.
All masquerade_extras would supply is the hooks/events that would be useful to rules.

Whether or not Masquerade's maintainers want that integration in this suite or elsewhere is up to them.