Adding syntax highlighter

Description :

Add the possibility to use synthax highlighting for code block that use the "fenced" syntax :

    ```
    I'm a block of code
    ```
    ```QML
    I'm a (QML) highlighted block of code
    ```

User can use the following highlighters:

• KSyntaxHighlighter
• chroma
• pygments

But other could easily be added.

This also expose the backed in styles of the highlighter to the user.

This would be considered a "plugin" and is disabled by default.