Implement `features`

This feature was outlined in #385 [1], and documentation has been included as
part of this commit.

[1]: https://github.com/rust-lang/cargo/issues/385#issuecomment-53917539
auto merge of #509 : alexcrichton/cargo/features, r=wycats

This feature was outlined in #385 [1], and documentation has been included as
part of this commit.

[1]: https://github.com/rust-lang/cargo/issues/385#issuecomment-53917539

Closes #385