New mode for incremental slides

[szpak]

Incremental slides are very useful. However sometimes it could be preferred to show everything (the end slide with all point/elements), but gray out everything but the elements that are new on the current incremental slide (to not keep the audience suspended, but still emphasis what is the currently discussed point).

Sample markdown:

 - point 1
  - 1b
--
 - point 2
--
 - point 3

The classic incremental slides rendering. Step 1:

 - point 1
  - 1b

Step 2:

 - point 1
  - 1b
 - point 2

Step 2:

 - point 1
  - 1b
 - point 2
 - point 3

The proposed (optional - enabled on demand) incremental slides rendering (color are managed in CSS based in a class added by remark to mark which elements are current (were shown in the current step), which were already current in the previous steps and which will be current in the next step(s).

Step 1:

 - point 1 - black
  - 1b - black
 - point 2 - gray
 - point 3 - gray

Step 2:

 - point 1 - gray or black - depending on CSS
  - 1b - gray or black
 - point 2 - black
 - point 3 - gray

Step 3:

 - point 1 - gray or black
  - 1b - gray or black
 - point 2 - gray or black
 - point 3 - black

It is somehow related to #294 (possibly similar implementation in remark), but the end result is completely different.