- refactoring of render overwriting

- added the capability to add css class and id to images
- added the capability to add css class and id to links
- rearanged `guide -> helpers` to have an image section

Author: arelstone

docs/helpers.md

@@ -81,7 +81,9 @@ You will get `<a href="/demo/">link</a>`html. Do not worry, you can still set ti
   - [ ] bim
   - [ ] lim
 
-## Image resizing
+## Image
+
+### Resizing
 
 ```md
 ![logo](https://docsify.js.org/_media/icon.svg ':size=50x100')
@@ -96,6 +98,18 @@ You will get `<a href="/demo/">link</a>`html. Do not worry, you can still set ti
 ![logo](https://docsify.js.org/_media/icon.svg ':size=100')
 ![logo](https://docsify.js.org/_media/icon.svg ':size=10%')
 
+### Customise class
+
+```md
+![logo](https://docsify.js.org/_media/icon.svg ':class=someCssClass')
+```
+
+### Customise ID
+
+```md
+![logo](https://docsify.js.org/_media/icon.svg ':id=someCssId')
+```
+
 ## Customise ID for headings
 
 ```md