Make plugins less noisey and more stateful

[cooperlees]

• Only initialize if the plugin is enabled in config
• Move all plugins to store their loaded state as a static / class variable

Fixes #195 #193 #147

[codecov]

Codecov Report

Merging #197 into master will decrease coverage by 0.05%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master     #197      +/-   ##
==========================================
- Coverage   81.53%   81.47%   -0.06%     
==========================================
  Files           9        9              
  Lines         926      934       +8     
  Branches      138      141       +3     
==========================================
+ Hits          755      761       +6     
- Misses        145      147       +2     
  Partials       26       26

Impacted Files	Coverage Δ
src/bandersnatch/filter.py	98.21% <100%> (+0.17%)	⬆️
src/bandersnatch/mirror.py	93.01% <20%> (-0.8%)	⬇️

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