Provide possibility to customize the bodyParser configuration

[plc-dev]

Is your feature request related to a problem? Please describe.
I needed a way to increase the request size limits, which I could not do via custom middleware, as bodyParser is being set in the server by ltijs already, and I found no way to override the default "limit"-setting.

Describe the solution you'd like
Exposing a set of options to individualize the bodyParser configuration.

Describe alternatives you've considered
Using a custom middleware function or setting the configuration via the lti.app object (both unsuccessful).

Additional context
@Cvmcosta, see PR if time allows