Add CSS Utility for auto width and height Properties

Currently bootstrap only provides 4 variations of `width` and `height` css utilities, which are:

1. 25%
2. 50%
3. 75%
4. 100%.

Here, I'm adding `auto` to the list. This is useful when we need to reset the width/height value
back to the browser default value at some point.

docs/4.0/utilities/sizing.md

@@ -6,13 +6,14 @@ group: utilities
 toc: true
 ---
 
-Width and height utilities are generated from the `$sizes` Sass map in `_variables.scss`. Includes support for `25%`, `50%`, `75%`, and `100%` by default. Modify those values as you need to generate different utilities here.
+Width and height utilities are generated from the `$sizes` Sass map in `_variables.scss`. Includes support for `25%`, `50%`, `75%`, `100%`, and `auto` by default. Modify those values as you need to generate different utilities here.
 
 {% example html %}
 <div class="w-25 p-3" style="background-color: #eee;">Width 25%</div>
 <div class="w-50 p-3" style="background-color: #eee;">Width 50%</div>
 <div class="w-75 p-3" style="background-color: #eee;">Width 75%</div>
 <div class="w-100 p-3" style="background-color: #eee;">Width 100%</div>
+<div class="w-auto p-3" style="background-color: #eee;">Width auto</div>
 {% endexample %}
 
 {% example html %}
@@ -21,6 +22,7 @@ Width and height utilities are generated from the `$sizes` Sass map in `_variabl
   <div class="h-50 d-inline-block" style="width: 120px; background-color: rgba(0,0,255,.1)">Height 50%</div>
   <div class="h-75 d-inline-block" style="width: 120px; background-color: rgba(0,0,255,.1)">Height 75%</div>
   <div class="h-100 d-inline-block" style="width: 120px; background-color: rgba(0,0,255,.1)">Height 100%</div>
+  <div class="h-auto d-inline-block" style="width: 120px; background-color: rgba(0,0,255,.1)">Height auto</div>
 </div>
 {% endexample %}
 

scss/_variables.scss

@@ -127,7 +127,8 @@ $sizes: (
   25: 25%,
   50: 50%,
   75: 75%,
-  100: 100%
+  100: 100%,
+  auto: auto
 ) !default;
 
 // Body