Fix Icon Positions
Fix CSON positioning in tree view
Fix support for Image files (png, jpg etc)

They had the class ‘icon-file-media’
Make all tabs icons line up; Position all tree icons in the centre

[Closes #36] [Closes #38]
Fix hidden icon being chopped off and position
Reposition image icon in tab
Fix icon for README.md in TreeView and Tab

Needs a special class in the Tree View and when you have the Markdown
Preview open.
Merge pull request #33 from richrace/fix-icon-positions

Fix Icon Positions