Ensure config dir is writable

[bschreck]

Issue: on some environments (namely, some automatic CI environments) the home directory is not writable. Featuretools thus fails to install because we attempt to create a ~/.featuretools folder.

This PR resolves this problem by checking if the config folder is writable, and if not, it creates a temporary folder instead. It also allows a user to specify which folder to use for config via an environment variable ($FEATURETOOLS_DIR).

Much of this code was modified from the IPython source:

https://github.com/ipython/ipython/blob/master/IPython/utils/path.py (get_ipython_dir())

and

https://github.com/ipython/ipython/blob/master/IPython/paths.py (get_home_dir())

Lastly, it changes config_yaml.txt to a proper yaml file: config.yaml

[codecov-io]

Codecov Report

Merging #135 into master will decrease coverage by 0.08%.
The diff coverage is 72.54%.

@@            Coverage Diff             @@
##           master     #135      +/-   ##
==========================================
- Coverage   88.76%   88.68%   -0.09%     
==========================================
  Files          75       76       +1     
  Lines        7803     7845      +42     
==========================================
+ Hits         6926     6957      +31     
- Misses        877      888      +11

Impacted Files	Coverage Δ
...utational_backend/test_calculate_feature_matrix.py	99.55% <100%> (+0.43%)	⬆️
featuretools/config.py	71.08% <61.76%> (-7.77%)	⬇️
featuretools/tests/utils_tests/test_config.py	93.33% <93.33%> (ø)
featuretools/utils/gen_utils.py	66.66% <0%> (+2.22%)	⬆️

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[kmax12]

any reason for shifting order? I'd think we want things to fail faster

[bschreck]

My thought was that you'd want to know if your code passed the tests regardless of whether it passed linting. That is a more important indicator of how your branch is doing.

[bschreck]

In this case in particular, the lint error was masking the actual testing error that I cared about, and was not able to reproduce locally

[kmax12]

gotcha. this change makes sense to me

[kmax12]

Looks good. Merging