Change baseline default of prediction_breakdown() to be consistent with broken()

[kevinykuo]

This change:

• Removes the default of the baseline parameter of prediction_breakdown(). This is so that the defaults agree with the underlying breakDown::broken() function. This is a potentially breaking if users were depending on the hardcoded baseline = "Intercept", but I'd argue consistency with broken() should be the expected behavior.

Closes #39

[codecov-io]

Codecov Report

Merging #43 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #43   +/-   ##
=======================================
  Coverage   89.35%   89.35%           
=======================================
  Files          12       12           
  Lines         263      263           
=======================================
  Hits          235      235           
  Misses         28       28

Impacted Files	Coverage Δ
R/single_prediction.R	80% <100%> (ø)	⬆️

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[pbiecek]

Thanks!
This is more consistent.
What worries me is the lack of backward compatibility.
Will add a more direct warning to the NEWS.md