Update Jekyll on Windows info

Help Windows users install Jekyll

Mention that Python needs to be in PATH for Pygments to work and thus jekyll not fail.

Update less.js to v1.5.1.

Build process: use module "load-grunt-tasks" to load grunt tasks

This fixes an error with `&:extend(.clearfix all)` when using the customizer.

enable strictMath in grunt-contrib-less config

more LESS strictMath fixes

<3 @seven-phases-max for helping to confirm & debug this
X-Ref: https://github.com/less/less.js/issues/1725

improve compliance with LESS's strictMath

Drop grunt-recess for grunt-contrib-less

Conflicts:
	Gruntfile.js
	package.json

more nesting in .list-group-item-variant mixin

Conflicts:
	Gruntfile.js
	dist/css/bootstrap-theme.min.css
	dist/css/bootstrap.min.css

Original discussion:
https://github.com/less/less.js/issues/1437#issuecomment-21383639.

Since we’re switching to `grunt-contrib-less`, we can take advantage of
newer LESS features than what RECESS supported. Included in that is the
ability to `:extend`, and not only that, but `:extend(.mixin-name
all)`. By doing so, we remove duplicate CSS for all our elements that
were being clearfix-ed.

Fixes #8947, #8968, #8991, #9257, #9268, #9291, #9430, #9604, #9686,
#9929, #10731, #10793, #11305, #11498, #11533, #11570, #11604, #11652.

(dem issues, tho)

Conflicts:
	dist/css/bootstrap.min.css