Remove unimplemented config vars
Use default optimization level in Cabal config
Use default options when bootstrapping Cabal
Default to not cutting GHC
Support Cabal 1.20.0.3
Support GHC 7.8.3
Remove unhappy workaround
Support building customized sandboxes
Work around overly cautious failure logic
Rename prepared to precompiled
Improve notation
Improve failure logic
Add customize script helper and example