Add support for extending configurations and plugins

[politician]

If I use a NPM module, that contains identifiers or use certain terms that are false positives, the only way to fix this is to manually add them to skipWords.
If someone else uses the same NPM module, he will have to do the same.

Why not pool the work the same way DefinitelyTyped started for the Typescript community?

Plugins could be named following this convention: spellcheck-npm-name

Examples:

• spellcheck-react for react
• spellcheck-babel__core for @babel/core (reference)

It could also support a spellcheck option in package.json so package authors could use it to point to a local spellcheck config/dictionary the same way the types entry works.