Implement features

[alexcrichton]

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

Closes #385

[alexcrichton]

Oops, wrong title!

[sfackler]

cargo or feature?

[sfackler]

Is the definition of a dependency-less feature allowed? For example, rust-openssl has support for SSLv2 hidden behind a cfg flag: https://github.com/sfackler/rust-openssl/blob/4a823242abd136417939a49ebb121533381c01ca/src/ssl/mod.rs#L51. That kind of use case might be worth mentioning in the docs.

[alexcrichton]

Aha, thanks for reminding me @sfackler! That caught a few bugs here and there, and I've put an explicit mention in the documentation now as well.

Some interesting changes to resolve:

• Cyclic dependencies are now explicitly disallowed.
• A test for two dependencies having the same name has been modified, because I believe this case to be impossible. The dependencies map in TOML is a hash map, so it's impossible to have two keys with the same name.

[andrew-d]

@alexcrichton Any update on this?

[alexcrichton]

I need to work on getting this rebased past the implementation of the registry, which will require some finesse that is not currently implemented.

[alexcrichton]

Rebased, r? @wycats

[wycats]

r+

I think this will need a bit of cleanup at some point, but I'm ok with it as is.