Use a variable for the default font-weights

Created by: sbaechler

Right now Bootstrap uses the hardcoded font weights bold and normal.

Websites might use different weights such as medium or thin or use light as default weight instead of normal.

If Bootstrap used settings variables for $font-weight-normal and $font-weight-bold the user could easily override the default values or use numerical weights.

This would also help #18605 (closed).

Those two lines could be added to the _variables.scss:

$font-weight-normal: normal;
$font-weight-bold: bold;

All occurrences of font-weight: normal; would be replaced by font-weight: $font-weight-normal. The same applied for font-weight: bold;.

Foundation uses a similar approach.

I could create a pull-request for this.