Create new stylesheets for optional builds and add them to Gruntfile tasks

scss/_normalize.scss

@@ -98,7 +98,7 @@ a {
   &:active {
     outline: 0;
   }
-  @include hover {
+  &:hover {
     outline: 0;
   }
 }

scss/bootstrap-flex.scss

@@ -0,0 +1,8 @@
+// Bootstrap with Flexbox enabled
+//
+// Includes all the imports from the standard Bootstrap project, but enables
+// the flexbox variable.
+
+$enable-flex: true;
+
+@import "bootstrap.scss";

scss/bootstrap-grid.scss

@@ -0,0 +1,57 @@
+// Bootstrap Grid only
+//
+// Includes relevant variables and mixins for the regular (non-flexbox) grid
+// system, as well as the generated predefined classes (e.g., `.col-4-sm`).
+
+
+//
+// Variables
+//
+
+// Grid system
+//
+// Define your custom responsive grid.
+$grid-breakpoints: (
+  // Extra small screen / phone
+  xs: 0,
+  // Small screen / phone
+  sm: 34em,
+  // Medium screen / tablet
+  md: 48em,
+  // Large screen / desktop
+  lg: 62em,
+  // Extra large screen / wide desktop
+  xl: 75em
+) !default;
+
+// Number of columns in the grid.
+$grid-columns:               12 !default;
+
+// Padding between columns. Gets divided in half for the left and right.
+$grid-gutter-width:          1.5rem !default;
+
+
+// Container sizes
+//
+// Define the maximum width of `.container` for different screen sizes.
+
+$container-max-widths: (
+  sm: 34rem,    // 480
+  md: 45rem,    // 720
+  lg: 60rem,    // 960
+  xl: 72.25rem  // 1140
+) !default;
+
+
+//
+// Grid mixins
+//
+
+@import "variables";
+
+@import "mixins/clearfix";
+@import "mixins/breakpoints";
+@import "mixins/grid-framework";
+@import "mixins/grid";
+
+@import "grid";

scss/bootstrap-reboot.scss

@@ -0,0 +1,10 @@
+// Bootstrap Reboot only
+//
+// Includes only Normalize and our custom Reboot reset.
+
+@import "variables";
+@import "mixins/hover";
+@import "mixins/tab-focus";
+
+@import "normalize";
+@import "reboot";