Passing a non default instance of `bottle.Bottle` to `eel.start()` does not work

[Sentinent]

Eel version
0.14.0

Describe the bug
eel.start() uses bottle.route() to add eel routes, which only adds routes to the default Bottle application. Thus, if a non default instance of bottle.Bottle is passed into eel.start(), the passed in instance will not have eel routes, and will not work properly.
Current implementation

To Reproduce

import bottle
import eel

app = bottle.Bottle()
@app.route('/custom')
def some_route():
    return 'Hello World!'
eel.init('web')
eel.start('index.html', port=0, app=app)

The webserver will 404 on any of the eel routes (/eel.js, /eel, any files in web, etc).

Expected behavior
Eel will start the bottle server with the routes needed for eel to work.

Additional context
Original issue #211 was fixed by #212, but #229 caused a regression of the original issue.

More discussion in PR #573