Flag: library path

[steveklabnik]

This issue is part of #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?

[steveklabnik]

This seems pretty important.

[hjr3]

Similar to #135 (comment) I am not sure if --library-path is needed. cargo supports configuring library paths through build.rs, which rustdoc honors when generating the analysis.