Fix for #955: Stop weird behavior when adding percentages

What do you expect the result of `10% + 10%` to be? Well, everyone in
the world would say `20%`. Stylus thinks it should be `11%`.
Basic sourcemaps support
Updated History with the latest changes
Merge branch 'pr/1664' into rc-0.48

* pr/1664:
  Fix for #955: Stop weird behavior when adding percentages
Updated History
merge feature-source-maps
Allow globals and functions to be set in options
Added tests for #1653
Updated History
History up
0.48.0