Add option to verify local security.txt file

[BartG95]

It would be nice to able to verify a local security.txt file on my computer.

Positive consequences:

• I can check the file before uploading.
• It be used with CI.

Negative consequences:

• The path can't be verified.

Workaround as long as this is not implemented:

• A local web server.

How this could be implemented:

• Add an option is_local to the constructor, and use it branch between a HTTP request and just opening a file.

[DigitalTrustCenter]

With version 0.9.2 it is now possible to test local files by adding the is_local flag to True and passing a local path as the url parameter. This will only validate the content of the file given