2.x: Added option to try to sanitize a modified subject if verification fails

[dodmi]

My mail provider prepends mail subjects with *** UNCHECKED *** or *** SPAM *** or similar. This invalidates the dkim signature.
I added an option on the 2.x branch, which removes any text of the form *...* or [...] from the beginning of the subject and retries verification.

[lieser]

Changes LGTM in general.

But was wondering what your reasoning for what to allow was,

• Why do you allow adding [...]? Is this for mailing lists that change the subject but don't modify the body?
• Why do you not enforce something to be between the *...* or [...]? I.e. change the .* in the middle to .+.

Just as a reference in case I will some day port this to the current release, note that this relates too #202.

[dodmi]

• Why do you allow adding [...]? Is this for mailing lists that change the subject but don't modify the body?

Well, I came over additions to the subject, which looked like [Some text here] (indeed a mailing list) and *** Some text here *** (the spam filter of my mail provider). And I tried to find a way to get DKIM verifier to be able to get the right result.

• Why do you not enforce something to be between the *...* or [...]? I.e. change the .* in the middle to .+.

I changed this to .+, thank you