Allow print/write with multiple newlines

[estk]

This PR resolves #2990.
It should now still disallow the single case and allow the case where there are more than one trailing newlines. Also, I updated the message for the lint to specify that it is for single newlines.

[Manishearth]

r+

[Manishearth]

(waiting on travis, which may also just fail because of rust versions until we land #2941)

[estk]

Thanks @Manishearth , yeah additionally I noticed that clippy is not compiling on master for me locally since 328fea3.

evan:~/rust/rust-clippy# cargo test
   Compiling clippy_lints v0.0.212 (file:///Users/evan/rust/rust-clippy/clippy_lints)
error[E0004]: non-exhaustive patterns: `GlobalAsm(_)` not covered
   --> clippy_lints/src/utils/mod.rs:920:11
    |
920 |     match def {
    |           ^^^ pattern `GlobalAsm(_)` not covered
error: aborting due to previous error
For more information about this error, try `rustc --explain E0004`.
error: Could not compile `clippy_lints`.