Whitelist plugin request: `jekyll-last-modified-at`

Hi,
After looking for a way to display the last modification date of pages / collections, I found a good Jekyll plugin that do that: [jekyll-last-modified-at](https://github.com/gjtorikian/jekyll-last-modified-at).

**_But_**, since the plugin isn't whitelisted I cannot use it directly with the Github Pages native flow. I forced to build the Jekyll site and then push the result to Github.

**_So_**, I decided to create this "issue" to discuss and know if it was possible to whitelist the plugin.

If there is a better alternative / way / plugin to display the last modification date of a page (without adding it manually via front matter) I'm interested.

### Before submitting an issue, please be sure to

- [x] Read [the contributing instructions](https://github.com/github/pages-gem/blob/master/docs/CONTRIBUTING.md#contributing-to-the-github-pages-gem)
- [x] Update to the latest Gem version (run `bundle update github-pages`)

### This issue affects

- [x] The site generated by GitHub Pages
- [ ] Building sites locally

*The GitHub Pages Gem is intended to help users replicate the GitHub Pages build environment locally. If your issue affects both the hosted version and the version previewed locally, you may be better suited reporting [seeking support in other forums](https://github.com/github/pages-gem/blob/master/docs/SUPPORT.md).*