Implement cfg-based target-specific dependencies

Implement cfg-based target-specific dependencies

This commit is an implementation of [RFC 1361][rfc] which is an extension of
Cargo's `target` section in `Cargo.toml` to allow the use of `#[cfg]`-like
expressions for target-specific dependencies. Now that the compiler has been
extended with `--print cfg` each invocation of Cargo will scrape this output and
learn about the relevant `#[cfg]` directives in play for the target being
compiled. Cargo will then use these directives to decide whether a dependency
should be activated or not.

This should allow definition of dependencies along the lines of:

    [target.'cfg(unix)'.dependencies]
    [target.'cfg(target_os = "linux")'.dependencies]
    [target.'cfg(windows)'.dependencies]

Which is much more ergonomic and robust than listing all the triples out!