Add support for multiple unit sets and output de-dupped sorted selectors

This plugin can now be used with an array of unit sets:

    var rework    = require('rework'),
        pureGrids = require('rework-pure-grids');

    var css = rework('').use(pureGrids.units([5, 24])).toString();

The above will create a de-dupped collection of grid units for 5ths- and
24ths-based units.
Bump version to v0.0.1