add Printf support

[aplavin]

It's very convenient to print intervals (and other objects) with flexible number formatting. Printf is reasonably extensible, and this PR adds Printf support for intervals. + corresponding tests.
FWIW, there are a few packages with this kind of Printf support already: https://juliahub.com/ui/Search?type=code&q=Printf.fmt.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.21%. Comparing base (13c76e1) to head (53c8489).
⚠️ Report is 19 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #201      +/-   ##
==========================================
- Coverage   99.11%   98.21%   -0.90%     
==========================================
  Files           6        7       +1     
  Lines         225      280      +55     
==========================================
+ Hits          223      275      +52     
- Misses          2        5       +3     

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[aplavin]

Not sure what codecov job complains about, all new code seems covered

[aplavin]

bump...