Fix NullCallableString() when called with no or multiple arguments
Add testcase for NullCallableString
Bring readme up to date.
Explain the new behavior to myself. Remove 1 test assertion. No non-test behavior changes. Bump version to 1.5.1.

Remove one test assertion that already occurs almost verbatim in test_formatting_functions_without_tty().
Merge Vitja Makarov's NullCallableString tty-less parametrization fix. Close #31.