Add leading zero to sub minute duration formatting

[MikeDuister]

Currently values below 60 are formatted without a leading minute unit. This PR proposes that the minute counter should not collapse but should display "0", in turn the seconds counter must always be of double digits.

Formatting of values equal to or larger than 60 remains unchanged.

seconds	previous formatting	current formatting
0	0	0:00
10	10	0:10
60	1:00	1:00
3600	1:00:00	1:00:00

This change reflects the behaviour of SoundCloud

[lbrndnr]

Hi Mike,
Thanks for the pull request and sorry for the massive delay in response.
Leading zeros are definitely a good idea, but I think it's much easier/cleaner when implemented using a DateComponentsFormatter, as I did in Nutshell. If you still feel like fixing this, I'm very happy to merge it :)

[lbrndnr]

Oh just noticed you were already using DateComponentsFormatter, my bad. I have now fixed this in the latest commit using your code, but ditching the regular expression, just to keep it as simple as possible.