Update breakpoints.js

[CSS variable names must always be prefixed with "var-"](https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_variables#What_are_CSS_Variables). I left in backwards-compatibility with the old naming convention, but I would suggest you consider removing it.
Update README.md
Merge pull request #1 from dvdln/var-names

Update variable naming convention to valid CSS