Summary

Adds the ability to override the baseUrl through the module options. Closes #4

What are the specific steps to test this change?

1. Set the APOS_BASE_URL environment variable -OR - baseUrl configuration value
2. Run node app @apostrophecms/sitemap:print and see that the sitemap includes the baseUrl in page URLs
3. Add a different baseUrl value in the module options object
4. Run node app @apostrophecms/sitemap:print again, and see that the module's config option is used instead

What kind of change does this PR introduce?

(Check at least one)

• Bug fix
• New feature
• Refactor
• Documentation
• Build-related changes
• Other

Make sure the PR fulfills these requirements:

• It includes a) the existing issue ID being resolved, b) a convincing reason for adding this feature, or c) a clear description of the bug it resolves
• The changelog is updated
• Related documentation has been updated
• Related tests have been updated

If adding a new feature without an already open issue, it's best to open a feature request issue first and wait for approval before working on it.

Other information:
This feature is helpful when running Apostrophe in docker or behind a proxy, where the baseUrl may internally be localhost.