Signal: `did-finish-load` URL and Title retrieval

[jmercouris]

Due to the way that we handle signals right now we have the following workflow:

1. Some event is triggered by cl-electron
2. This event invokes a lisp thread
3. The lisp thread may further inquire for information from cl-electron

By the time we get to step 3, the data that we inquire for may be stale. In this case, we're trying to fetch the URL and Title for a given did-finish-load event. The only way to do this properly would be to implement a custom did-finish-load handler that grabs the title and URL and returns it to the Lisp thread as data.