Fixed a bug that #31 preview always happens if there's an active region.
Replace mark-active with (use-region-p)
Changes to come closer to Emmet support, to allow for better consistency of behavior.

Major changes include:

  * Parser is changed to use Emmet's bracket syntax for properties
  * now uses point as the right bound of the expression
  * searches for left bound from right bound. This is poorly executed as of now