Integrate an exclude flag

[tboerger]

Enable an option to exclude glob patterns like it's available for the
file flag, that way we can exclude for example .sha256 files from being
signed while the rest can be signed in specific folders.

[tboerger]

An example could be like this:

  gpg-sign:
    image: plugins/gpgsign:1
    pull: true
    secrets: [ gpgsign_key, gpgsign_passphrase ]
    detach_sign: true
    files:
      - dist/release/*
    excludes:
      - dist/release/*.sha265