Added the ability to ignore specific patches from dependencies

[mortenson]

This PR adds a new feature, "patches-ignore", which allows a Composer package to ignore specific patches from dependencies. There are a number of use cases for the composer-patches project that this enhances:

1. I use a different more recent version of a dependency, and now a patch isn't applying.
2. I have a more up to date patch than my dependency, and want to use mine instead of theirs.
3. A dependency's patch adds a feature to a project that I don't need.
4. My patches conflict with a dependency's patches.

This is also a critical feature for Drupal distributions that require other distributions, as distributions typically contain loads of patches for Drupal modules. For the "child" distribution, using a different version of a module means that parent patches will not apply, which could break the build or cause unintended consequences. With this feature, a child distribution could ignore a patch that applied to an older version of a dependency, use a newer version of that dependency, and add a newer patch that applies cleanly.

Here's an example composer.json I wrote which requires the Lightning distribution for Drupal, and ignores a patch which adds a feature that conflicts with an edge case in our distribution (Demo Framework). This is use case 3 mentioned above:

{
  "minimum-stability": "dev",
  "repositories": [
    {
      "type": "composer",
      "url": "https://packagist.drupal-composer.org"
    },
    {
      "type": "vcs",
      "url": "https://github.com/mortenson/composer-patches"
    }
  ],
  "require": {
    "drupal/lightning": "dev-8.x-1.x",
    "cweagans/composer-patches": "dev-master as 1.5.0"
  },
  "extra": {
    "patches-ignore": {
      "drupal/lightning": {
        "drupal/panelizer": {
          "This patch has known conflicts with our Quick Edit integration": "https://www.drupal.org/files/issues/2664682-49.patch"
        }
      }
    }
  }
}

[grasmash]

Great feature, we also need the ability to do this.

Pinging @cweagans for visibility.

[cweagans]

@mortenson @grasmash Sorry for the slow response here. 👍 from me.

[mariagwyn]

Any chance this can be documented in the README?