feat(version): throw error when webpack 2 is installed (#157)

Closes #152

BREAKING CHANGE: webpack v2 is not supported and now there will be a
message when you're using webpack v2 with webpack-validator.

feat: Allow performance.hints in webpack config (#150)

* feat: Allow performance.hints in webpack config

Webpack 2.1.0-beta.28 adds `performance.hints` (which take boolean
values) as a way to enable/disable performance warnings. (The first
performance warning is about bundle size.)

* fix: Fix name of an import

The test file for the `performance` property should import the test utils module using `utils` in

lowercase but instead imported `Utils` in uppercase. This worked fine on my machine because my

filesystem is case-insensitive but fails in Travis because its filesystem is case-sensitive.

fix(docs): improve maintenance wording (#147)

To make the expectations more clear.

fix(README): phrase the note abouth webpack v2 in a slightly better w…

…ay (#145)

fix(devServer): add setup property to devServer options

Add missing setup property to devServer options.
See https://webpack.github.io/docs/webpack-dev-server.html#api for setup
option

Close #143

fix(deps): cross-env upgraded (#135)

Merge pull request #134 from kentcdodds/pr/coverage

test: ignore line that is difficult to simulate

fix(devtool): allow `false` as `devtool` value (#128)

fix: Set default empty object for validate options (#121)

Default option object to keep caller from having to pass empty object during function invocation

none

fix: fix exposed API (#119 regression)

It should expose a function like before now. The previous commit changed
it to export an object accidentally and this commit restores the old
behavior.