Refactor the formatter to an interface and rename it

This change refactors the formatter to be an interface called Printer.
`NewApp` will use the functionality of `ScreenPrinter`,  which is what I
refactored the old `Formatter` class into.

There is a new class the implements the `Printer` interface called
`MemoryPrinter`, which will simply store the `groups` as they were
passed to it.  This will allow tests in `app_test.go` to become much
more powerful, since `App` no longer needs to be tightly coupled to
printing to the screen.

1 files changed, 9 insertions, 0 deletions

diff --git a/memory_printer.go b/memory_printer.go
new file mode 100644
index 0000000..62eaa83
--- /dev/null
+++ b/memory_printer.go
@@ -0,0 +1,9 @@
+package todolist
+
+type MemoryPrinter struct {
+	Groups *GroupedTodos
+}
+
+func (m *MemoryPrinter) Print(groupedTodos *GroupedTodos, printNotes bool) {
+	f.Groups = groupedTodos
+}