Add new commenting engines: Coral and Remark42

[igor-baiborodine]

This discussion is related to the pull request submitted by @mzch so that other users, if they want to integrate either Coral or Remark42 commenting engines, can profit from @mzch work, which is available in this branch: https://github.com/Lednerb/bilberry-hugo-theme/tree/idea/coral-remark42-integration.

For more details, check this commit: 1f20a97.