Expand and collapse folders recursively

Directories may be expanded or collapse recursively by holding `alt` when using
the keyboard or mouse. Incorporates pull request #88. Addresses issues #19 and #88.
To help users understand the difference between the `tree-view:expand-directory` and `tree-view:expand-directories` commands (which also look very similar), what do you think about renaming `tree-view:expand-directories` to something that mentions `recursive`? For example, `tree-view:recursive-expand-directory`. (+ same thing for the collapse commands.)