V2 pull request

Required
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- [x] - document plugins overview
- [x] - document less.render change
- [x] - review options/usage documentation
- [x] - review source map options once changed
- [x] - write upgrade guide (remember to point at clean css plugin)
- [x] - add some sentences about how to use less in a different environment

Nice to have
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- [x] - document file managers
- [ ] - document visitors
- [ ] - document post processors
- [ ] - recommend use of autoprefixer?