Add support for --cache

Hi! This has been discussed before: #99

However there are 3 problems with that solution:

• Cache is not persisted to disc
• It involves effort on the user, where one would expect just to set up a simple cache: true variable
• Eslint command line already does it in a well defined .eslintcache

Thank you!

[adametry]

While ESLint’s CLI has the responsibility of finding and reading files for linting, gulp-eslint relies on the Gulp stream to manage file sources. If a file should not be linted, it should not be passed to gulp-eslint in the stream.

The cached-lint example demonstrates how caching could be managed for a gulp stream; though, is not intended to be a direct implementation of ESLint’s CLI cache option, nor is it the only way to avoid needless file linting.

Fortunately, it is possible to persist a file/content cache to disc in a gulp stream (see gulp-cache-money or gulp-file-cache). However, I’m not aware a gulp plugin based on file-entry-cache (the cache package used by ESLint). If another file-based cache plugin doesn’t work in its place, perhaps creating a gulp plugin based on file-entry-cache would be useful with gulp-eslint as well as many other gulp plugins.

Though cache:true would be an easy option to configure, its simplicity is due to assumptions that must be made about the environment and use case. ESLint’s CLI acts as an independent tool, run directly against the file system. Because of this, a simple --cache option for a file-based cache is a stable and practical solution. On the other hand, Gulp’s virtual files may come from a range of sources and may be composed of a series of tools/plugins. For a plugin, it is uncertain whether a virtual file’s origin (if there is one) will match it’s destination (if there is one) or how the content may be transformed along the way. However, caching can be considered when composing a gulp task, where the expectations are certain.

[acierto]

You can find here a solution how to overcome that issue: https://dzone.com/articles/how-to-improve-performance-of-eslint-in-gulp