Added External Plugins Support

[danielsagi]

Description

Officially added support for plugins to be loaded. By using Pluggy

The hook specifications are centered at: kube_hunter/plugins/hookspecs.py

In order to implement your plugin, use the hookimpl decorator from kube_hunter.plugins and implement one of the following:

• parser_add_arguments - Add an argument
• load_plugin - Run code right after argument parsing

To load your plugin module you should install your package like this:

from setuptools import setup

setup(
    name="kube-hunter-mypackage",
    install_requires="kube-hunter",
    entry_points={"kube_hunter": ["mypackage = my_module"]},
    py_modules=["my_module"],
)

Current functionality was not impacted.

Main changes in the code:

• changed the argument parsing mechanism, the old function is now registered as a hook for the parsing steps, and receives an already initialized ArgumentParser object
• load_plugin is called, essentially calls all registered plugins, passing them the parsed args object, so they can have access to their custom argument. (which is not at the main static Config object)

Contribution Guidelines

Please Read through the Contribution Guidelines.

Fixed Issues

closes #321

"BEFORE" and "AFTER" output

BEFORE

AFTER

Contribution checklist

• [x] I have read the Contributing Guidelines.
• [ ] The commits refer to an active issue in the repository.
• [ ] I have added automated testing to cover this case.

Notes

[codecov[bot]]

Codecov Report

Merging #357 into master will increase coverage by 1.10%. The diff coverage is 91.30%.

@@            Coverage Diff             @@
##           master     #357      +/-   ##
==========================================
+ Coverage   58.11%   59.22%   +1.10%     
==========================================
  Files          40       42       +2     
  Lines        2039     2060      +21     
==========================================
+ Hits         1185     1220      +35     
+ Misses        854      840      -14     

Impacted Files	Coverage Δ
kube_hunter/conf/parser.py	76.00% <66.66%> (+76.00%)	:arrow_up:
kube_hunter/plugins/__init__.py	100.00% <100.00%> (ø)
kube_hunter/plugins/hookspecs.py	100.00% <100.00%> (ø)
kube_hunter/core/events/handler.py	90.09% <0.00%> (-1.00%)	:arrow_down:

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