Update eslint and enable linting of docstrings

[jbeezley]

This updates to eslint v3. Technically, v4 is out as of yesterday, but our peer dependencies haven't been updated yet. There are a number of new rules (mostly whitespace) that needed to be fix. Thankfully most of the changes only required running eslint --fix ..

The main purpose of this PR is to add linting of our docstrings. The rules I choose are designed to enforce the style of the docstrings added in map.js in #698. I have the rules set to warnings for now until we get around to fixing them all, which could take a while.

I also made a PR against eslint to add enforcement of parameter and return value descriptions (eslint/eslint#8716). If and when this is merged, we can enforce our docstring conventions there as well.

[codecov-io]

Codecov Report

Merging #706 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #706      +/-   ##
==========================================
+ Coverage   95.25%   95.27%   +0.02%     
==========================================
  Files          83       83              
  Lines        8980     8980              
==========================================
+ Hits         8554     8556       +2     
+ Misses        426      424       -2

Impacted Files	Coverage Δ
src/mapInteractor.js	96.12% <0%> (+0.28%)	⬆️

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[manthey]

I'm fine with this change, though I wish the --fix process did a nicer job.

[jbeezley]

Now that I look at it again, I think I'll change some of the new options to reduce the number of changes here. Rebase incoming.

[jbeezley]

I found there are some big changes to how indentation is handled in 4.0. It also provides more flexibility in configuration. I reverted the eslint update for now. I'll make another pass at it once our peer dependencies are all compatible with 4.0.

[jbeezley]

I'm getting all sorts of random errors on travis about missing node modules during the build. I had tried to upgrade npm, but that didn't seem to help, so I reverted that change.

[manthey]

I'll approve this again, since it changed substantially since the first time.