Support //! style comment as docstring for crate

[lbolla]

See discussion in #566.

For crates whose docstrings are in lib.rs, Racer's complete-with-snippet returns an empty list when completing the crate name itself. Compare with crates which instead use mod.rs, which behave correctly.

E.g. env_logger

[kngwyu]

Thanks for reporting, and could you please add for crate to the issue name?
I didn't know racer supports it for local modules (sorry).

[kngwyu]

Closed via #827