feat(validation): add option to exclude a list of commits by sha

This adds a new configuration option, `excluded-commits`, which allows
the user to provide a list of commits (full SHA1; 40 characters) that
will be skipped during validation runs.

This is useful for instance when having to merge branches from other,
less strictly checked repo's.

docs/configuration.md

@@ -30,6 +30,7 @@ initial-development: false  # OPTIONAL, defaults to `true`
 | `allowed-branches` | `.*` | A regex specifying from which branch(es) releases and Git tags are allowed to be created |
 | `initial-development` | `true` | A boolean indicating that this project is still under _initial development_. During this state, any commit message containing a breaking change will result in a `MINOR` version bump. |
 | `sdkver-create-release-branches` | `false` | For SdkVer versioning scheme only: push a new branch if an RC or release build is performed on a non-release branch. If this config value is boolean `true`, the branch shall be of the form `release/N.N`. If this value is set to a string, it shall be used as the branch name prefix and appended with the major and minor release numbers, e.g. config value  `"rel/"` results in a branch named `rel/N.N`. |
+| `excluded-commits` | `[]` | An optional list of (40-character) SHA1 hashes representing commits (and their ancestry) that should not be checked. For example, when merging another repo/branch with non-conventional-commits, add the SHA of that repo/branch's HEAD to the list, which allows it to pass the validation action. |
 
 > :bulb: By default `commisery-action` will search for the file `.commisery.yml`. 
 You can specify a different file with the `config` input parameter.