Let the help command work consistently everywhere

This adds a dummy help command so that it's usage can be documented with docopt!

Also adds help flags to all of the subcommands that were missing them. Without
that `cargo help sub-command` shows Invalid Argument before the usage text.
Remote `unused` lint

It's causing tons of not-very-helpful errors that are breaking the build.
auto merge of #670 : bkoropoff/cargo/unused-everywhere, r=alexcrichton

I'm not sure what changed, but unused value lints were popping up everwhere when I tried to build today.

This turns on the `if let` feature since it allows rewriting a lot of calls to `Option::map` that were only being used for their side effects into a clean form.  Cargo seems as good of a place as any to dogfood it.
auto merge of #667 : jakerr/cargo/help-help, r=alexcrichton

This adds a dummy help command so that it's usage can be documented with docopt! This lets `cargo help help` work.

Also adds help flags to all of the subcommands that were missing them. Without
that `cargo help sub-command` shows Invalid Argument before the usage text.
Clean up some uses of `Option::map` only for its side effects
auto merge of #672 : bkoropoff/cargo/option-map-cleanup, r=alexcrichton

This reduces syntactic noise and is a good use case for `if let`, which I turned on.