Support more markdown actions

[jvorcak]

Backend for this is ready so it's very easy to add basic markdown actions that wraps text.

• [ ] Insert code ``
• [ ] Underline

The way to do this is to add more methods like this and insert SVG icons to icons folder https://github.com/webscopeio/react-markdown-editor/blob/master/src/Toolbar.js#L53 https://github.com/webscopeio/react-markdown-editor/blob/master/src/Toolbar.js#L6