`output_timeout` should be configurable

[sbrugman]

The variable output_timeout is now a hardcoded constant:

https://github.com/computationalmodelling/nbval/blob/232d2496a01a5a5c96fe98b27977ae5505a15f5e/nbval/plugin.py#L336

For testing large outputs, such as notebook widgets, the 5 seconds are not sufficient. Making the variable configurable would solve that issue.