Allow enforcement of trailing slashes on urls and links

[jesseleite]

This PR adds support for enforcing trailing slashes on urls and links in generated SEO meta and in the sitemap. This is useful on server hosts that bias towards trailing slashes (ie. Netlify expects trailing slashes for SEO).

Todo

• Add opt-in config
  • We're not going to offer a config for this at the SEO Pro level. If you need this for statamic/ssg purposes, you can enable it in ssg's config, or across your whole app via URL::enforceTrailingSlashes() in a provider, etc.
• Flesh out test coverage
  • Everything seems to be working when testing manually though! 💅
• Add to DOCUMENTATION.md
  • Explain that if you're wanting trailing slashes for statamic/ssg reasons (ie. Netlify), that you should probably use ssg's dedicated config or enable for your app via URL::enforceTrailingSlashes().

References #387
References statamic/cms#11840
References statamic/ssg#203