Give kernel providers opportunity to load configuration

[kevin-bates]

This change adds a new method to KernelProviderBase names load_config(). When KernelFinder initializes itself with a set of providers, it calls this method on each provider passing the hosting application's Config instance, if available. If an application is not currently available or not initialized, the method is still called with config=None in cases where providers may not be hosted within an application and/or can perform their own loading of configuration - however that is defined by the provider.

Fixes #17

[takluyver]

My thinking was that the application would pass in its config to KernelFinder, rather than relying on getting the global application singleton. No strong preference, though.

[kevin-bates]

Thanks for the comment. I don't have a strong preference either, although I think that if we left it up to the hosting application, then for those applications that pass config=None (or don't specify a parameter) in KernelFinder's constructor, it seems the unconditional call on the provider's load_config() would be less intuitive. Also, because 'config' in this case is not just a dictionary (but rather a Config class instance), I think it's less error prone to manage that ourselves. That said, I could be swayed by counter arguments as well.

[codecov-io]

Codecov Report

Merging #20 into master will increase coverage by 0.59%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #20      +/-   ##
==========================================
+ Coverage   70.45%   71.05%   +0.59%     
==========================================
  Files          27       27              
  Lines        2004     2045      +41     
==========================================
+ Hits         1412     1453      +41     
  Misses        592      592

Impacted Files	Coverage Δ
jupyter_kernel_mgmt/tests/test_discovery.py	95.68% <100%> (+1.71%)	⬆️
jupyter_kernel_mgmt/discovery.py	64.22% <100%> (+2.83%)	⬆️

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