Allow to generate lower case header references through the config

This makes the references consistent/compatible with GitHub,
GitLab, Pandoc and many other tools.

This behavior can be enabled in config.json with:

```
"linkifyHeaderStyle": "gfm"
```

Signed-off-by: hoijui <hoijui.quaero@gmail.com>

public/js/extra.js

@@ -833,7 +833,11 @@ const linkifyAnchors = (level, containingElement) => {
     if (header.getElementsByClassName('anchor').length === 0) {
       if (typeof header.id === 'undefined' || header.id === '') {
         // to escape characters not allow in css and humanize
-        const id = slugifyWithUTF8(getHeaderContent(header))
+        let id = slugifyWithUTF8(getHeaderContent(header))
+        // to make compatible with GitHub, GitLab, Pandoc and many more
+        if (window.linkifyHeaderStyle !== 'keep-case') {
+          id = id.toLowerCase()
+        }
         header.id = id
       }
       if (!(typeof header.id === 'undefined' || header.id === '')) {

public/js/lib/common/constant.ejs

@@ -5,4 +5,6 @@ window.version = '<%- version %>'
 
 window.allowedUploadMimeTypes = <%- JSON.stringify(allowedUploadMimeTypes) %>
 
+window.linkifyHeaderStyle = <%- JSON.stringify(linkifyHeaderStyle) %>
+
 window.DROPBOX_APP_KEY = '<%- DROPBOX_APP_KEY %>'