Add clean-script option

[developit]

This addresses the case outlined in #59.

/cc @scinos

---

preview of added doc section:

Clean up state between builds

For repositories or custom monorepo setups where files are modified in ways that are not reset by npm ci && npm run build, it may be necessary to define a custom "clean" script. This script resets any file modifications after the upstream (target) build ends and your PR code (HEAD) is checked out, but before installation of npm dependencies for HEAD:

name: Compressed Size
on: [pull_request]
jobs:
  build:
    runs-on: ubuntu-latest
    steps:
    - uses: actions/checkout@v2
    - uses: preactjs/compressed-size-action@v2
      with:
        repo-token: "${{ secrets.GITHUB_TOKEN }}"
+       clean-script: "clean"

// package.json
{
  "scripts": {
    // example - a simple nested node_modules setup:
    "postinstall": "cd packages && npm i",
    // between the two builds, we need to delete the inner node_modules:
    "clean": "rm -rf packages/node_modules"
  }
}

[github-actions]

Size Change: 0 B

Total Size: 75.5 kB

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Filename	Size
index.js	75.5 kB

_{compressed-size-action}