feat(button): added built-in support for rendering CircularProgress

If both the `@react-md/button` and `@react-md/progress` packages have
been installed and imported in the same file, the button will
automatically decrease the `CircularProgress`'s size to be more "dense"
and be the same size as an icon.

Author: mlaursen

packages/button/src/_mixins.scss

@@ -120,6 +120,9 @@
   }
   @include rmd-button-base;
   @include rmd-states-surface;
+  @if $rmd-button-circular-progress-size != null and mixin-exists(rmd-progress-theme-update-var) {
+    @include rmd-progress-theme-update-var(circular-size, $rmd-button-circular-progress-size);
+  }
 
   &--text {
     @include rmd-button-text;

packages/button/src/_variables.scss

@@ -2,6 +2,7 @@
 /// @group button
 ////
 
+@import '~@react-md/icon/dist/functions';
 @import '~@react-md/theme/dist/helpers';
 @import '~@react-md/theme/dist/mixins';
 @import '~@react-md/theme/dist/variables';
@@ -103,6 +104,46 @@ $rmd-button-floating-z-index: $rmd-utils-temporary-element-z-index !default;
 /// @type Number
 $rmd-button-floating-margin: 1.5rem !default;
 
+/// The size to use for a `CircularProgress` that exists as a child of the
+/// `Button`. The styles will only be created if this value is not `null`, the
+/// `@react-md/progress` package has been installed, and the
+/// `@react-md/progress`'s `mixin`s have been imported in the same file as the
+/// `@react-md/button`'s `mixin`s.
+///
+/// @example scss - Includes `CircularProgress` size<!-- no-compile -->
+///   @import '~@react-md/button/dist/mixins';
+///   @import '~@react-md/progress/dist/mixins';
+///   @import '~@react-md/utils/dist/mixins';
+///
+///   // can also manually set it to a different value if desired:
+///   // $rmd-button-circular-progress-size: 1.5rem;
+///
+///   // or @include react-md-button;
+///   @include react-md-utils;
+///
+/// @example scss - Does not include `CircularProgress` size<!-- no-compile -->
+///   // bad example since `@react-md/progress` was not imported
+///   @import '~@react-md/button/dist/mixins';
+///   @import '~@react-md/utils/dist/mixins';
+///
+///   // or @include react-md-button;
+///   @include react-md-utils;
+///
+/// @example scss - Opt-out of `CircularProgress` size<!-- no-compile -->
+///   @import '~@react-md/button/dist/mixins';
+///   @import '~@react-md/progress/dist/mixins';
+///   @import '~@react-md/utils/dist/mixins';
+///
+///   // manually set it to `null` to prevent it from being added
+///   $rmd-button-circular-progress-size: null;
+///
+///   // or @include react-md-button;
+///   @include react-md-utils;
+///
+/// @since 2.3.0
+/// @type String|Number
+$rmd-button-circular-progress-size: rmd-icon-theme-var(size) !default;
+
 /// A Map of floating position styles to create. Each key will be made into a
 /// className by `&--floating-#{$key}` and each value will parsed as a style
 /// map. If you want to remove positions, override this variable with only the