Normalize purge paths

[bradlc]

Fixes #107

chokidar and fast-glob both expect paths to use forward slashes.

Because we use path.resolve to define the purge paths in our tests, this results in Windows-style paths when running on Windows. This caused the issue described in #107.

This PR uses normalize-path to ensure that the paths passed to chokidar and fast-glob are what they expect.

Alternatives

We could use normalize-path in our tests, and make it clear in the docs that the purge paths should use forward slashes, but this PR means that people can do stuff like we did in the tests without worrying about it:

module.exports = {
  // currently this doesn't work on Windows
  purge: [path.resolve(__dirname, './index.html')],
}

[adamwathan]

Thanks! 👊