Flag: library path

This issue is part of https://github.com/steveklabnik/rustdoc/issues/125

-L/--library-path:

Using this flag looks like this:

$ rustdoc src/lib.rs -L target/debug/deps
$ rustdoc src/lib.rs --library-path target/debug/deps

If your crate has dependencies, rustdoc needs to know where to find them. Passing --library-path gives rustdoc a list of places to look for these dependencies.

This flag takes any number of directories as its argument, and will use all of them when searching.

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Should we keep this flag or not?