Support multiple units in format_timedelta()

[hartmutobendorf]

Issue #728 describes returning measures for several units when using format_timedelta().

E.g. instead of

10840 seconds
3 hours

return for depth=="fullest"

3 hours 0 minutes 40 seconds

and for depth=="full"

3 hours 40 seconds

Tests are passing, but I haven't tried contributing to babel before, and am not aware of any additional requirements beyond the few lines of source code in this PR. Feedback appreciated.

[codecov]

Codecov Report

Attention: Patch coverage is 69.23077% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 90.94%. Comparing base (40e60a1) to head (8683607).

Files	Patch %	Lines
babel/dates.py	69.23%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1066      +/-   ##
==========================================
- Coverage   90.99%   90.94%   -0.05%     
==========================================
  Files          26       26              
  Lines        4444     4453       +9     
==========================================
+ Hits         4044     4050       +6     
- Misses        400      403       +3     

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