Beta-2

Added

• Builder-based implementation for customizing the cargo rustc -- --print cfg command
• The --lib flag to the cargo rustc --lib -- --print cfg command as default (See #3)
• Support for specifying a toolchain via the CargoRustcPrintCfg builder type (See #1)
• Information to the module documentation about needing toolchains and targets installed (See #2)

Changed

• The Cfg::new method to the Cfg::host method (See #4)
• The Cfg::with_triple method to the Cfg::rustc_target method
• Documentation tests to reflect new API and more appropriate examples

Removed

• The Cfg::with_args method