Update checks based on latest webPreferences defaults

[ikkisoft]

We're now on Electron 7.x as stable. Since we built our checks - many webPreferences default configs have changed.

E.g. Since v5, nodeIntegration is disabled by default. Our current nodeIntegration checks will trigger a false positive if there's no explicit setting since we still assume that the default is on. I believe the same problem apply to many other checks.

[phosphore]

This is an important feature that we're still missing.
A few days ago @baltpeter tried to address this in #66, but since we are expecting a constant increase in complexity between different electron versions in the future, I would like Electronegativity to approach this in the most flexible and maintainable way.

This is my design proposal:

1. Build a JSON-based "defaults.json" file, containing the default webPreferences options for each version of Electron, possibly using a semantic versioning grammar. This file will be shipped with Electronegativity. The file could be generated programmatically by coding a script to instrument every Electron version (included in the releases.json file) and dump the webPreferences object. From then on we could just run the script whenever a new Electron version is released.
2. Since it is not always possible for the targeted project to include a neat package.json file containing the Electron version, Electronegativity should attempt to obtain the framework version from any package.lock/yarn.lock file. Alternatively, users should be able to set the exact Electron version used by the targeted application via command line arguments (e.g. --electron-version=8.1.2?).
3. Using (1) and (2) we will be able to provide each check with a defaults argument to use, containing the default webPreferences options for the Electron version used by the application.

This will help us eliminate many version-dependent decisions from every check.

[phosphore]

Thanks to @baltpeter (#66, #67) Electronegativity is getting smarter! 🎉 Things we still need to do before pushing a new version:

• Add some more tests Consider Electron version and update checks according to new defaults (#23, #58) #66 (comment)
• Update README.md and the documentation presenting the new -e flag
• Merge Don't fail in CSPGlobalCheck if CSP is invalid #68