feat: added credential_process option to config

[elyzov]

resolves #16

[kir4h]

Thanks @elyzov! I´ll try to take a look over the week-end

[codecov[bot]]

Codecov Report

Patch coverage: 78.94% and project coverage change: -0.73 :warning:

Comparison is base (f0d8850) 89.61% compared to head (9e5bfe8) 88.88%.

:exclamation: Current head 9e5bfe8 differs from pull request most recent head 61575cc. Consider uploading reports for the commit 61575cc to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #17 +/- ## ========================================== - Coverage 89.61% 88.88% -0.73% ========================================== Files 6 7 +1 Lines 260 279 +19 ========================================== + Hits 233 248 +15 - Misses 13 15 +2 - Partials 14 16 +2 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `88.88% <78.94%> (-0.73%)` | :arrow_down: | 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=Andr%C3%A9s+%C3%81lvarez#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://app.codecov.io/gh/kir4h/rvault/pull/17?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andr%C3%A9s+%C3%81lvarez) | Coverage Δ | | |---|---|---| | [internal/pkg/credential/process.go](https://app.codecov.io/gh/kir4h/rvault/pull/17?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andr%C3%A9s+%C3%81lvarez#diff-aW50ZXJuYWwvcGtnL2NyZWRlbnRpYWwvcHJvY2Vzcy5nbw==) | `78.94% <78.94%> (ø)` | |

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