[Feature] Nested Staticman Comment

[VincentTam]

Resolves #57. A minimised port of Beautiful Hugo's native Staticman support. I'm not expert in CSS, so I'll leave the that for any interested user(s).

Demo site on GitLab

Staticman now supports GitLab at https://vincenttam.gitlab.io/hyde-hyde-demo

Quick guide to test this pull request

If you're using a theme as a Git submodule (as recommended in Hugo's official tutorial), you may create a new (local/GitLab/etc) (private) repo for this theme and run these commands to get this PR merged against a testing branch staticman in the private repo. I denote the URL of this repo as the upstream.

$ git remote -v
upstream	https://github.com/htr3n/hyde-hyde.git (fetch)
upstream	https://github.com/htr3n/hyde-hyde.git (push)
... # other remote omitted
$ git checkout -b staticman # test on a new branch staticman
$ git fetch upstream pull/58/head # git pull will be rejected
$ git merge FETCH_HEAD # manually merge the this PR against branch staticman
$ cd <your-blog>
... # edit your .gitmodules with url="<repo-containing-staticman>" and branch = "staticman"
$ git submodule sync # inform Git the changes in .gitmodules
$ git submodule update --remote --recursive # switch to the HEAD of your cloned repo for the theme

Remarks

1. Staticman needs neither JavaScript nor CSS. (e.g. My minimal Jekyll + Staticman repo) I've ported the JavaScripts and loaded jQuery for notifying users for successful/failed comment submission. Please feel free to ask me to strip that off if you don't want that.
2. I've left aside comment Markdown preview, since that requires an additional script like Showdown. Not all users like the idea of loading an additional script. Interested users may add such script in the custom layout.