Nuclei - support for custom headers

[TheTechromancer]

Addresses https://github.com/blacklanternsecurity/bbot/issues/1029.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 91.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 92%. Comparing base (1e5ff17) to head (ad474f6).

Files	Patch %	Lines
...est/test_step_2/module_tests/test_module_nuclei.py	91%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## bbot-2.0 #1482 +/- ## ======================================== + Coverage 92% 92% +1% ======================================== Files 346 346 Lines 23710 23734 +24 ======================================== + Hits 21740 21773 +33 + Misses 1970 1961 -9 ```

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