Add accessor for the yq logger instance (#1013)

Allow consumers of yqlib to customize the logger instance.

Closes #432

Signed-off-by: Carolyn Van Slyck <me@carolynvanslyck.com>

Co-authored-by: Carolyn Van Slyck <me@carolynvanslyck.com>
Co-authored-by: Rob Ferguson <robert.j.ferguson@nasa.gov>

pkg/yqlib/lib_test.go

@@ -0,0 +1,10 @@
+package yqlib
+
+import "testing"
+
+func TestGetLogger(t *testing.T) {
+	l := GetLogger()
+	if l != log {
+		t.Fatal("GetLogger should return the yq logger instance, not a copy")
+	}
+}