Implement ExternalPrinterWriter

[xeruf]

To enable duplex output of log messages

Fixes #797

[xeruf]

Only Problem: When rustyline logs at debug level, the prompt somehow does not work. Fixed in my project by .filter(Some("rustyline"), LevelFilter::Warn)

[gwenn]

I am afraid that on incomplete / partial write, rustyline is going to append newline (\n) where they are not expected.
See

rustyline/src/edit.rs

Lines 324 to 326 in e4eca97

	if !msg.ends_with('\n') {
	self.out.write_and_flush("\n")?;
	}

[xeruf]

That is why the method was called println in the rustylinez fork, but that is not the main problem even though it is not quite ideal.
The readline function completely hangs when at debug level for rustyline, probably they somehow interfere with each other.

[gwenn]

Indeed, it should be named println.
But initially, there was no newline appended automatically until:
#629
#627