Flag: extern

[steveklabnik]

This issue is part of #125

--extern: specify a dependency's location

Using this flag looks like this:

$ rustdoc src/lib.rs --extern lazy-static=/path/to/lazy-static

Similar to --library-path, --extern is about specifying the location
of a dependency. --library-path provides directories to search in, --extern
instead lets you specify exactly which dependency is located where.

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Should we keep this flag or not?

[steveklabnik]

This is very important!

[euclio]

I don't think this is needed if we depend on cargo. cc #155

[steveklabnik]

That's true; I was thinking in the "cargo calls rustdoc" mode, not "rustdoc calls cargo"