Support crate features

Cargo supports Features, which, according to the [documentation](http://doc.crates.io/manifest.html#the-features-section) is used for the following:

1. Specifying conditional compilation options
2. Specifying optional dependencies
3. Specifying clusters of optional dependencies, other features of the current crates, and/or enabling features in dependencies of this crate.

(1) is currently supported via the `crate_features` attribute, which sets flags to enable different conditional compilation features via `cfg`. More discussion and design work is needed in order to support (2) and (3).

Read Cargo's manifest Features documentation and the existing crate_features attribute support first. Define the design needed for optional dependencies and feature groups or dependency features; done means those remaining Cargo feature behaviors are supported.

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