Support different assets.version file paths

[acobster]

As a Conifer user, when registering/enqueuing scripts or styles I want to be able to specify my own cache-busting mechanism or file, instead of using the hard-coded assets.version file path.

Acceptance Criteria:

• [x] Add support for passing an array with a file key as the $version param, which tells Conifer\Site which file to use instead of /path/to/theme/assets.version
• [x] If true is passed instead of an array, it should still look for assets.version (this is the current behavior)
• [x] If any other arg is passed, it should pass it on to the corresponding WP core fn (e.g. wp_register_script) without modification (this is the current behavior)
• [x] Update Site::get_assets_version() to look in the file passed in $version
• [x] Document the new array option in site.md

Example Usage

    $this->register_script(
      'my-script',
      'my-script.js',
      [],
      ['file' => 'my-assets-version.txt']
    );

This tells get_assets_version() to look for a path/to/theme/my-assets-version.txt file, instead of the default path/to/theme/assets.version.