Bootstrap v4.0.0-alpha.4 发布

Bootstrap v4.0.0-alpha.4 发布了，这个版本主要的变化是用一个小的”alpha patch” 来命名一个没有任何JS文件的npm包，并且对我们的 CSS 和 docs 进行了一些小小的改变。

#20130: Make .bg-inverse use the background variant mixin for proper text color.
#20333: Update progress bar to use $progress-border-radius instead of $border-radius.
#20387: Use $input-border-radius-* variables on .input-group-addons instead of $border-radius.
#20413: Remove unused $size parameter from grid mixin.
#20447: Make cards block level in columns.
#20469: Remove non ASCII character from table CSS.
#20485: Use rem units on Alert’s dismiss icon.
#20545: Removed the inconsistent margins between flex and non-flex modes from .media. Replaced them with spacing utils in our docs examples.
#20597: Removed unused variables in some grid mixins.
#20619: Ensure hover effects work on text utilities.
#20629: Update form checkbox and radio styles for validation; tweak docs to match.

#20282: Fix typo in Migration docs about Reboot and unit changes.
#20341: Update static tooltip demo in docs to use proper class names.
#20417: Import the flexbox utilities into the docs flex stylesheet to fix broken flex utility docs.
#20475: Update flexbox grid docs to show auto-sizing of sibling columns when using a centered, odd-count column.
#20476: Remove CDN coming soon notice from docs page.
#20484, #20489: Fix segmented button in input group examples.
#20505: Update migration guidelines for renamed table variants.
#20519: Add migration note about .panel-group.
#20551: Ensure progress component table of contents renders.
#20553: Properly size split dropup variation with modifier class.