Add customizable border radius to progressbar

[Come2Daddy]

This is a improvement.

$radius-rounded is not meant to be modified and is used directly to style progress bars. We need a dedicated variable to override this style if needed.

Proposed solution

Add $progress-border-radius into sass/elements/progress.sass with $radius-rounded as default value.

Tradeoffs

None

Testing Done

For each test.
Defined $progress-border-radius to some values (px/rem/%), then imported bulma.
Checked the border-radius, with built css file, over progress element : ok.

Did the same with jekyll for the docs.