Add a search command to cargo

[ghost]

This will currently call out to the registry service. In the future it may be sensible to add package descriptions to the git index so that search can be done offline.

Closes #925.

[ghost]

This is rebased on top of #956.

[alexcrichton]

Could you post a sample of the output as well? This looks great, thanks @jakub-!

[ghost]

@alexcrichton Of course!

[jakub@jakub cargo (cargo-search ✗)]$ ./target/cargo search foo
    Updating registry `https://github.com/rust-lang/crates.io-index`
hoare (0.1.1)          Design by contract style assertions for Rust
tinycdb (0.0.3)        Bindings to the TinyCDB C library (http://www.corpit.ru/mjt/tinycdb.html)
memcache (0.0.4)       memcached client for rust
lazy_static (0.1.1)    A macro for declaring lazily evaluated statics in Rust.
flate2 (0.0.3)         Bindings to miniz.c for DEFLATE compression and decompression exposed as Reader/Writer streams. Contains bindings for zlib, def…
json_macros (0.0.2)    Convenience macros for constructing JSON objects from literals.

Hmm, I'm not sure why we're getting all these results for foo but I presume some fields other than name and description get full text indexed. This makes the results a tad less useful but I think it's valuable to at least search in descriptions.

[ghost]

@alexcrichton Updated. Your comments got lost due to my rebase but I fixed the command --help and renamed ExistingCrate to Crate.

[alexcrichton]

r+, gonna wait on #956 though.

[ghost]

@alexcrichton r? if you're still happy with this? :-)