refactor(fileTree): assign node icons in LESS rather than script
refactor(splitter): default to halving extent of parent for each pane
style(fileTree): use default cursor for file tree
feat(fileTree): add file tree icons based on extension

- Update to latest FontAwesome which includes some custom file icons, audio/video/image/archive/etc..
- Pull the file extension from a node when generating its class list.
docs(README): add fontawesome attribution
feat(textEditorView): support specifying document mode
refactor(documentView): specify child extents once

- remove height/width 100% from child editors, and specify that any document editor child sized to avoid repetition.
feat(markdownEditorView): add basic editor for markdown files

- Use showdown to render markdown to HTML strings
- Double click to toggle between preview/edit mode.
- TODO: Bind to document data rather than url, so that changes made in editor are reflected in preview.
style(markdownEditorView): better than horrible style