feat: add successSearchQuery option

[raffis]

Current situation

semantic-relase spams all pull requests every created in a repository with a released comment.
This is quite problematic in older repositories with many prs merged.
As far as I can tell there is no way currently to exclude pr older than, or prs with certain labels.

Proposal

Add an option successSearchQuery which enables user to override the github query.

For example the following filter allows me to pre label all old pr's with released, and after introducing semantic-release
in my repo, old pr's don't get spammed with comments.

    [
      "@semantic-release/github",
      {
        "successSearchQuery": "repo:${owner}/${repo}+type:pr+is:merged -label:released"
      }
    ]

This change is fully backwards compatible as it falls back to the default query.

Note this achieves something similar as proposed in #360

[raffis]

Any feedback?