Default implementation of on_request_success and _failure lacks **kwargs declaration

[karol-brejna-i]

Description of issue / feature request

When implementing new (custom) client or trying to inject more metadata into results the user will usually need to introduce dedicated success/failure handlers that may extend the default list of parameters and/or fire success/failure events manually (probably with additional arguments).

Take this for example:
https://gist.github.com/karol-brejna-i/da6295a4d7bf25c2796fa95c622e7d80

Expected behavior

Users should be able to introduce additional request handlers that include extended parameter lists.

Actual behavior

Because of the way the default request handlers are implemented (

locust/locust/stats.py

Line 557 in 5e1ccc5

def on_request_success(request_type, name, response_time, response_length):

), Locust will fail in this case with something similar to:
https://gist.github.com/karol-brejna-i/4ab6b0a283a5cc56e4013acef31622f8

Environment settings (for bug reports)

• OS: doesn't affect the results; Docker, tested on Ubuntu 16.04, Windows 10
• Python version: 3.6
• Locust version: 0.8.1

Steps to reproduce (for bug reports)

Define additional handlers, fire request_success with additional arguments as described in https://medium.com/p/183d2ae4a4c2

Suggested solution

Making the implementation adhere to the following advice (https://docs.locust.io/en/latest/extending-locust.html):

It’s highly recommended that you add a wildcard keyword argument in your listeners (the **kw in the code above), to prevent your code from breaking if new arguments are added in a future version.

would solve the issue.