feat: added support for Disqus (#120)

* feat: added support for Disqus
* feat: allow disqus to be disabled through frontmatter

Set:
    bookDisableComments: true

On a page to disable disqus for the content.

README.md

@@ -24,6 +24,7 @@
 - Customisable
 - Zero initial configuration
 - Handy shortcodes
+- Optional support for Disqus
 
 ## Requirements
 
@@ -117,6 +118,10 @@ You can also see `yaml` example [here](https://github.com/alex-shpak/hugo-book/b
 # Always put it on the top of the configuration file, otherwise it won't work
 googleAnalytics = "UA-XXXXXXXXX-X"
 
+# (Optional) If you provide a Disqus shortname, comments will be enabled on
+# all pages.
+disqusShortname = "my-site"
+
 # (Optional) Set this to true if you use capital letters in file names
 disablePathToLower = true
 
@@ -190,6 +195,9 @@ bookHidden = true
 
 # (Optional) Set how many levels of ToC to show. use 'false' to hide ToC completely
 bookToC = 3
+
+# If you have enabled Disqus for the site, you can disable it for specific pages.
+bookDisableComments = true
 ```
 
 ### Partials