add option to include special files in the check for unused dependencies

[rouanw]

Example:

$ npm-check --specials=eslint,webpack

depcheck supports the concept of specials which are special parsers to check files other than your usual JavaScript source code. (See https://github.com/depcheck/depcheck#special.)

Current support includes npm scripts, eslint, webpack, babel and mocha. (See https://github.com/depcheck/depcheck/tree/master/src/special for the full list.)

This PR adds specials to the API and CLI and passes the value through to depcheck when checking for unused dependencies.

I've opened a PR in depcheck - depcheck/depcheck#206 - to make the list of available options clearer in the documentation.

[rouanw]

@amilajack / @LinusU - would you mind taking a look please?

[amilajack]

@rouanw is this supposed to save npm-check from having to guess the special deps?

[rouanw]

Hi @amilajack. No, it is basically a way to tell npm-check to also check for usages in some 'special' files. For example, your .eslintrc, webpack.config.js and your npm scripts, like test. This reduces the number of false positives when checking for unused dependencies.

The naming is unfortunately not very intuitive, but I stuck to using the depcheck naming to avoid having to maintain additional documentation and mapping code.