Added option to return sub-sitemap URLs in results (#161)

* Added option to return sub-sitemap URLs in results

* Add prepare script to run build before packaging

* fix: correct typo in SitemapperResponse interface and type definition

* feat: added SitemapperOptions.proxyAgent in sitemapper.d.ts

* Moving example to ReadMe

* Fixing typscript issue

* Update package-lock.json

* Removing prepare for now

* 3.3.3

---------

Co-authored-by: seantomburke <seantomburke@users.noreply.github.com>
Co-authored-by: GitHub Action <action@github.com>

Author: 3 people

README.md

@@ -84,6 +84,7 @@ You can add options on the initial Sitemapper object when instantiating it.
 - `exclusions`: (Array<RegExp>) - Array of regex patterns to exclude URLs from being processed
 - `fields`: (Object) - An object of fields to be returned from the sitemap. Leaving a field out has the same effect as `<field>: false`. If not specified sitemapper defaults to returning the 'classic' array of urls. Available fields:
   - `loc`: (Boolean) - The URL location of the page
+  - `sitemap`: (Boolean) - The URL of the sitemap containing the URL, userful if <sitemapindex> was used in the sitemap
   - `lastmod`: (Boolean) - The date of last modification of the page
   - `changefreq`: (Boolean) - How frequently the page is likely to change
   - `priority`: (Boolean) - The priority of this URL relative to other URLs on your site
@@ -132,7 +133,9 @@ const sitemapper = new Sitemapper({
   fields: {
     loc: true,
     lastmod: true,
-    changefreq: true,
     priority: true,
+    changefreq: true,
+    sitemap: true,
   },
+});
 ```