Allow to pass 't' argument of 'clip.show' as 'HH:MM:SS' format

[mondeja]

• t argument of clip.show method accepts HH:MM:SS time formats.
• Add test for internal tool and proper documentation to preview methods.

• I have provided code that clearly demonstrates the bug and that only works correctly when applying this fix
• I have added suitable tests demonstrating a fixed bug or new/changed feature to the test suite in tests/
• I have properly documented new or changed features in the documentation or in the docstrings
• I have properly explained unusual or unexpected code in the comments around it

[coveralls]

Coverage increased (+0.3%) to 77.928% when pulling 1da8466 on mondeja:show-t-string into 4d59f79 on Zulko:master.