add support to --start and --end time formats like mm:ss or hh:mm:ss or dd:hh:mm:ss

[valerio-bozzolan]

This changeset allows multiple TIME formats for --start and --end.

• ss seconds (example: 1500 or 1500.3)
• mm:ss minutes and seconds
• hh:mm:ss hours minutes and seconds
• dd:hh:mm:ss days, hours, minutes and seconds

This changeset also introduces a small license section in the README with some credits to the original author.

I've tested this and it seems to me that it has backwards compatibility.

NOTE: A very useful feature for the future is to add another parameter similar to --end but that time counts from the beginning rather than from the end. So one can enter the minutes that your favorite player shows when you identified your timeline points.

[jhenstridge]

I've pushed the clean-ups I suggested and merged the feature. Thank you for your contribution!