Include/Exclude for custom properties

## New Feature

Similar to the current behavior for labels, it would be nice to be able to exclude certain custom properties.

```
custom-properties:
include:
- name: ruleset-tier
value: strict

exclude:
# ignore (don't delete) the following custom-properties
- name: team
```

This way certain custom-property values that are organization-wide can be enforced, while others that are required to be set, but have different value per repo, will be left untouched.