Add Option to Disable Target DNS Regex

[TheTechromancer]

In scans with a lot of targets, excavate is taking up a lot of CPU time and slowing down scans significantly. This is because a regex is generated for every target in order to extract matching subdomains from HTTP_RESPONSE, etc.

Because this is preventing ASM scans from finishing in a timely manner, I've added this option which allows that regex feature to be disabled.

This is a temporary fix and will be removed again once @liquidsec finishes his excavate overhual.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 92%. Comparing base (ab9df4b) to head (ce75049). Report is 1 commits behind head on stable.

Files	Patch %	Lines
bbot/scanner/scanner.py	67%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## stable #1435 +/- ## ====================================== - Coverage 92% 92% -0% ====================================== Files 321 321 Lines 20685 20688 +3 ====================================== - Hits 19014 19007 -7 - Misses 1671 1681 +10 ```

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