Add short-with-failures format

[dnephin]

Closes #72

Adds a new format, similar to short, that prints test output as soon as a test fails. Useful when a test suite runs for a long time and you want to see errors before the run ends.

[wendorf]

Works great! Thanks for doing this. The help text needs updating, though:

diff --git a/main.go b/main.go
index cd9b7c9..81cc97e 100644
--- a/main.go
+++ b/main.go
@@ -65,11 +65,12 @@ Flags:
 		flags.PrintDefaults()
 		fmt.Fprint(os.Stderr, `
 Formats:
-    dots              print a character for each test
-    short             print a line for each package
-    short-verbose     print a line for each test and package
-    standard-quiet    default go test format
-    standard-verbose  default go test -v format
+    dots                 print a character for each test
+    short                print a line for each package
+    short-verbose        print a line for each test and package
+    short-with-failures  print a line for each test and package
+    standard-quiet       default go test format
+    standard-verbose     default go test -v format
 `)
 	}
 	flags.BoolVar(&opts.debug, "debug", false, "enabled debug")

[dnephin]

Good catch, thanks!