Add a custom editor for notes preview

[viddo]

The current problem is that opening a text-editor (even in pending state) is quite costly, this is an already known issue, atom/fuzzy-finder#5

Considering the preview should focus on text notes, and the goal of the preview is to give a quick glance of the content + matched strings the best solution is probably to implement a minimalistic editor that fulfills the following requirements:

• Preview editor should show content of note
• Highlight matched strings; scroll to first match if outside the visible viewport.
• Upon hitting Enter or focusing on the preview window (e.g. by clicking pane), the editor should be replaced by the default text editor.
• When a note is unselected (e.g. hitting Esc) the preview should be closed.
• Visually it can look similar to the default text editor, but doesn't need all the advanced features (grammar, word wrap etc.), so should be sufficient of rendering a simple HTML element with a note's content.
• Reuse preview when changing selected note.