Allow running dependency tests with the specific webpack version

[DarkoKukovec]

Related issue: #24

• [x] Link webpack to dependency
• [x] Ignore missing examples if options.test is set
• [x] Run tests
• [x] Support for Jest
• [x] Update docs
• [x] Expose in squawk
• [x] Add tests
• [x] Fix issue when using npm - https://github.com/npm/npm/issues/16685
• [x] Check why example fails
• [x] Add timeout option

Test command: node index.js --webpack=webpack/webpack#master --dependency=https://github.com/webpack-contrib/extract-text-webpack-plugin/archive/master.tar.gz --test="npm test" --example-dir=example

Breaking change: If example-dir is not set, examples will not be checked.