Aligning particle rotation

Rotation can improve many particle effects. We are forced to pick and choose the values we want to use for the particle system in our project because, frankly, I could write an entire book on particle systems. Instead of having rotation value ranges, like we did earlier for the particle's scale, we are going to have a single flag that allows the user to choose whether the particle system is going to align its rotation with the emission velocity vector. I find this to be a pleasant effect. The user will make this decision with an id="align_rotation" checkbox.

Here is what the HTML code looks like:

<label class="ccontainer"><span class="label">align rotation:</span>
 <input type="checkbox" id="align_rotation" checked="checked">
 <span class="checkmark"></span>
 </label>
 <br/>