Rename _name to custom_name for pipelines

[jeremyliweishih]

fixes #533.

[codecov]

Codecov Report

Merging #650 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #650   +/-   ##
=======================================
  Coverage   98.97%   98.97%           
=======================================
  Files         136      136           
  Lines        4692     4694    +2     
=======================================
+ Hits         4644     4646    +2     
  Misses         48       48           

Impacted Files	Coverage Δ
...l/pipelines/classification/random_forest_binary.py	100.00% <100.00%> (ø)
...pelines/classification/random_forest_multiclass.py	100.00% <100.00%> (ø)
evalml/pipelines/classification/xgboost_binary.py	100.00% <100.00%> (ø)
...lml/pipelines/classification/xgboost_multiclass.py	100.00% <100.00%> (ø)
evalml/pipelines/pipeline_base.py	98.25% <100.00%> (ø)
evalml/pipelines/regression/random_forest.py	100.00% <100.00%> (ø)
evalml/tests/pipeline_tests/test_pipelines.py	99.64% <100.00%> (+<0.01%)	⬆️

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[jeremyliweishih]

Documentation doesn't clearly show that custom_name alters name. This is tracked in #652.

[dsherry]

Is this the only usage of custom_name in our unit tests? That's surprising. Could you double-check that we have adequate coverage? For example we could add a test here which calls assert TestDefinedNamePipeline.custom_name == "Cool Logistic Regression" or something. What does it show up as when we try to call pipeline.custom_name for a pipeline which doesn't define a custom name? Good to have this sort of coverage in case we change the impl in the future.

[jeremyliweishih]

If we call for a pipeline that doesn't have custom_name it should error out. I can change it to return None.

[dsherry]

Looks great! I added a couple notes but otherwise ready to 🚢