Better host inheritance

[TheTechromancer]

FINDING and VULNERABILITY will now inherit their host, path, and url attributes from their parents if they aren't specified.

Addresses #1636.

[codecov]

Codecov Report

Attention: Patch coverage is 92.50000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 93%. Comparing base (3172528) to head (e735daa).
Report is 30 commits behind head on dev.

Files	Patch %	Lines
bbot/core/event/base.py	92%	2 Missing ⚠️
bbot/core/engine.py	0%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##             dev   #1666   +/-   ##
=====================================
+ Coverage     93%     93%   +1%     
=====================================
  Files        341     341           
  Lines      25926   25932    +6     
=====================================
+ Hits       23893   23906   +13     
+ Misses      2033    2026    -7     

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