Customize formatting, mappings, and coloring

 * Switch task indicators to `( )` / `(X)`
 * Integrate with syntax colors rather than specifying static ones
 * Change toggling to two-state by default (to avoid removing task
   when cycling)
 * Add mapping for Control-Enter to use default toggling
 * Set up Control-Enter to create new tasks but only toggle on/off after
 * Add mapping for Control-Shift-Enter for three-state toggling (between
   on, off, and no task marker)
 * Add mappings for insert mode toggling
 * Add Control-Command-Enter / Control-Shift-Command-Enter mappings for
   toggling a task and moving to the next row