Add option to save request and response headers

[malexmave]

Some use cases require getting access to request and response headers of page loads. This PR adds that feature. To enable it, set your SCAN_MODULE_OPTIONS in your config file to include 'RequestsExtractor.saveHeaders': True for the chromedevtools scan module. Minimal config example:

SCAN_MODULE_OPTIONS = {'chromedevtools': {'RequestsExtractor.saveHeaders': True}}

This PR also fixes a problem with loading a config file using the --config file.py option, where the scanner would crash due to a missing cast of the parameter string to a Path object.

[hprid]

Thank you for your pull request. Here are some remarks regarding the changes:

• Try to avoid cleanup commits like 5461bf7 in the same pull request. Instead rebase your code before and squash those commits into one if they are actually only one small change. git rebase -i HEAD~3 or similar is your friend.
• When writing comments, focus on the why instead of the what. Having a comment say that you cast something if the next code line is a cast does (5461bf7) not contribute to the understanding. I personally would leave out the comment, since it is absolutely fine to pass the config_file as an Path object; it is not required to be a string (although all calls are currently passing a string). Python will happily leave your Path object untouched if you “cast” it to Path again.
• Split logical changes into several pull requests, i.e., do not fix a minor bug together with a feature addition. I will not care for this pull request, but please keep it in mind for future ones.
• I would rename the option RequestsExtractor.saveHeaders to RequestsExtractor.save_headers to be consistent with Python's style guide, i.e., CamelCase for class names and snake_case for attributes.
• One remark regarding b411149, that is not relevant any more: Try to not include lines in a try block which do not raise a relevant exception that is meant to be caught.

After reworking your pull request should include two commits (config_file fix + RequestExtractors.save_headers option) that I will happily rebase into master.

[malexmave]

Thanks for the feedback. I rebased and squashed the relevant commits and changed the parameter name.