DMARC Filter: allow to define subject regexp

[matclab]

Some DMARC report mail subject have a prefix before "Dmarc Report" in the subject and are not checked by the plugin. This commit allows the user to define the suject regexp in the config file.

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (74c91d8) 47.02% compared to head (0ed9386) 47.02%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #332 +/- ## ======================================= Coverage 47.02% 47.02% ======================================= Files 30 30 Lines 1076 1076 ======================================= Hits 506 506 Misses 570 570 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `47.02% <0.00%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=afewmail#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://codecov.io/gh/afewmail/afew/pull/332?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=afewmail) | Coverage Δ | | |---|---|---| | [afew/filters/DMARCReportInspectionFilter.py](https://codecov.io/gh/afewmail/afew/pull/332?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=afewmail#diff-YWZldy9maWx0ZXJzL0RNQVJDUmVwb3J0SW5zcGVjdGlvbkZpbHRlci5weQ==) | `25.00% <0.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=afewmail). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=afewmail)

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[GuillaumeSeren]

Hey @matclab , thank you for the patch.