[WIP] Add PRINTLN_EMPTY_STRING lint.

I have no idea of how I could properly check that there is really an
empty string argument in a `println` invocation.
See `HACK` in print.rs for details.

Author: MaloJaffre

clippy_lints/src/lib.rs

@@ -530,6 +530,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry) {
         partialeq_ne_impl::PARTIALEQ_NE_IMPL,
         precedence::PRECEDENCE,
         print::PRINT_WITH_NEWLINE,
+        print::PRINTLN_EMPTY_STRING,
         ptr::CMP_NULL,
         ptr::MUT_FROM_REF,
         ptr::PTR_ARG,