update harp to 0.12.1

Update to a new version of Harp, which has lots of new features, bugfixes, and updated preprocessors:

- EJS is now at version 1.0.0, which includes many minor fixes
- Node-sass is now at version 0.8.4, which adds basic @extends support and other fixes that move libsass closer towards parity with Sass 3.2.
- Stylus is now at version 0.42.3 with blocks support and more (similar to Sass’ @extends)
- LESS is now at version 1.7.0, which includes many fixes and some new features like added support for rulesets in variables and passed to mixins to allow wrapping

More info on the Harp blog: http://harpjs.com/blog/v0-12-1-boilerplates