Add accuracy as an objective

[angela97lin]

Closes #294 after #373 is merged by adding Accuracy as a standard metric.

[codecov]

Codecov Report

Merging #559 into 373_objective_classes will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@                  Coverage Diff                   @@
##           373_objective_classes     #559   +/-   ##
======================================================
  Coverage                  98.77%   98.77%           
======================================================
  Files                        127      127           
  Lines                       4553     4559    +6     
======================================================
+ Hits                        4497     4503    +6     
  Misses                        56       56           

Impacted Files	Coverage Δ
evalml/objectives/__init__.py	100.00% <ø> (ø)
evalml/objectives/utils.py	94.44% <ø> (ø)
evalml/objectives/standard_metrics.py	99.50% <100.00%> (+0.01%)	⬆️
evalml/tests/objective_tests/test_objectives.py	96.29% <100.00%> (ø)

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