Added stacked_icon helper

[jtrost]

This feature is from issue #59

Usage

<%= stacked_icon(
  icon("camera", "", class: "fa-stack-1x"), # The first icon to stack
  icon("ban", "", class: "fa-stack-2x text-danger"), # The second icon to stack
  class: "fa-lg" # Additional classes for fa-stack
) %>

Output

<span class="fa-stack fa-lg"><i class="fa fa-camera fa-stack-1x"></i><i class="fa fa-ban fa-stack-2x text-danger"></i></span>

[houndci-bot]

Line is too long. [82/80]

[jvanbaarsen]

@supercodepoet Is this something you would be interested in? If so what do you think need to be changed for this to be merged? I can take over this PR if needed.

[ignisf]

@supercodepoet, apart from the stylistic issues @houndci found in the PR, is there another reason you believe it should not be merged?