Closes #97

I investigated #97 and discovered that addBabelPresets works similarly to addBabelPlugins in that it spreads its arguments and then maps them. This means that passing an array of presets instead of a flat list of arguments will result in an array nested one level deeper than expected.

Correct usage example:

module.exports = override(
  ...addBabelPresets(
    [
      "@babel/env",
      {
        targets: {
          browsers: ["> 1%", "last 2 versions"]
        },
        modules: "commonjs"
      }
    ],
    "@babel/preset-flow",
    "@babel/preset-react"
  )
);

Changes

• added a note in readme.md to not pass an array to addBabelPresets
• removed the surrounding array from the addBabelPresets arguments in the readme.md code example
• installed jest
• added tests for addBabelPreset and addBabelPresets to guarantee their usage matches this PR
• added tests for the rest of the babel functions
• replaced npm test script with jest call
• added coverage/ to .gitignore for jest --coverage usage