Verbose debug logging via the ConsolePrinter and printer package. (#404)

This:
 * adds a global -v flag for verbosity
 * refactors the "printer" package to have a DefaultPrinter and package
   functions that call it, similar to net/http's DefaultServeMux
 * adds printer tests
 * moves current users of Debugf to Printf
 * moves most users of the "log" package to use "printer"
 * demotes noticably noisy log messages to "Debugf", like "IGNORE"-
   and "NO_PURGE"-related messages

pkg/printer/printer_test.go

@@ -0,0 +1,47 @@
+package printer
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// TestDefaultPrinter checks that the DefaultPrinter properly controls output from the package-level
+// Warnf/Printf/Debugf functions.
+func TestDefaultPrinter(t *testing.T) {
+	old := DefaultPrinter
+	defer func() {
+		DefaultPrinter = old
+	}()
+
+	output := &bytes.Buffer{}
+	DefaultPrinter = &ConsolePrinter{
+		Writer:  output,
+		Verbose: true,
+	}
+
+	Warnf("warn\n")
+	Printf("output\n")
+	Debugf("debugging\n")
+	assert.Equal(t, "WARNING: warn\noutput\ndebugging\n", output.String())
+}
+
+func TestVerbose(t *testing.T) {
+	output := &bytes.Buffer{}
+	p := ConsolePrinter{
+		Writer:  output,
+		Verbose: false,
+	}
+
+	// Test that verbose output is suppressed.
+	p.Warnf("a dire warning!\n")
+	p.Printf("output\n")
+	p.Debugf("debugging\n")
+	assert.Equal(t, "WARNING: a dire warning!\noutput\n", output.String())
+
+	// Test that Verbose output can be dynamically enabled.
+	p.Verbose = true
+	p.Debugf("more debugging\n")
+	assert.Equal(t, "WARNING: a dire warning!\noutput\nmore debugging\n", output.String())
+}