Add docs for test formatter

test/utils.py

@@ -3,6 +3,9 @@ from psh.commands import BaseCommand
 from io import StringIO
 
 class TestFormatter(BaseCommand):
+    """Formatter useful for tests. Instead of printing to stdout, it
+    stores any output inside a stringio buffer. This can be retrieved
+    with the get_data method."""
 
     def __init__(self, *args, **kwargs):
         super(TestFormatter, self).__init__(*args, **kwargs)