Add all objectives currently supported by LightGBM

[hcho3]

Add all objectives currently supported by LightGBM. Notably, all regression objectives now offers an option sqrt. If this option is set, then the final output is given by f(x) = sign(x) * x**2 where x is the margin score.

Closes #94
Closes #98

[codecov-io]

Codecov Report

Merging #215 into mainline will decrease coverage by 0.03%.
The diff coverage is 66.66%.

@@              Coverage Diff               @@
##             mainline     #215      +/-   ##
==============================================
- Coverage       83.55%   83.52%   -0.04%     
- Complexity         44       46       +2     
==============================================
  Files              92       92              
  Lines            6664     6700      +36     
  Branches           42       50       +8     
==============================================
+ Hits             5568     5596      +28     
- Misses           1072     1079       +7     
- Partials           24       25       +1     

Impacted Files	Coverage Δ	Complexity Δ
src/compiler/native/typeinfo_ctypes.h	48.00% <41.66%> (-2.00%)	0.00 <0.00> (ø)
runtime/python/treelite_runtime/util.py	94.73% <71.42%> (-5.27%)	0.00 <0.00> (ø)
src/frontend/lightgbm.cc	83.73% <75.00%> (-0.56%)	0.00 <0.00> (ø)
src/compiler/native/pred_transform.h	80.88% <83.33%> (+0.23%)	0.00 <0.00> (ø)
src/compiler/pred_transform.cc	68.00% <100.00%> (+1.33%)	0.00 <0.00> (ø)
...src/main/java/ml/dmlc/treelite4j/java/DMatrix.java	69.76% <0.00%> (+13.95%)	9.00% <0.00%> (+2.00%)

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[wphicks]

Looks great! Just one typo, but besides that I think we're good to go.

[wphicks]

LGTM!

[hcho3]

@wphicks Thanks for reviewing!