printer: use bstr

This starts the usage of bstr in the printer. We don't use it too much
yet, but it comes in handy for implementing PrinterPath and lets us push
down some platform specific code into bstr.

grep-printer/src/lib.rs

@@ -70,6 +70,7 @@ fn example() -> Result<(), Box<Error>> {
 
 #[cfg(feature = "serde1")]
 extern crate base64;
+extern crate bstr;
 extern crate grep_matcher;
 #[cfg(test)]
 extern crate grep_regex;