Extract tasks into separate packages

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Tasks

• js: https://github.com/unic/estatico/blob/develop/gulp/js/default.js (include preset-env, check out HMR)
• js:lint (based on eslint, add fix option)
• js:modernizr: https://github.com/unic/estatico/blob/develop/gulp/js/modernizr.js (possibly replace with Webpack loader instead)
• js:test: Based on Puppeter and QUnit (for now): Use chrome instead of PhantomJS jonkemp/gulp-qunit#31 (comment)
• js:mocks: https://github.com/unic/estatico/blob/develop/gulp/js/mocks.js
• css: https://github.com/unic/estatico/blob/develop/gulp/css/default.js → compare to webpack afterwards @alinamelnyk
• css:lint (based on sass-lint or stylelint) @Wymowny
• css:colors: Probably replace with https://github.com/Updater/node-sass-json-importer
• css:fonts: https://github.com/unic/estatico/blob/develop/gulp/css/fonts.js @orioltf
• media:svgsprite: https://github.com/unic/estatico/blob/develop/gulp/media/svgsprite.js @drowka
• [low prio] media:iconfont: https://github.com/unic/estatico/blob/develop/gulp/media/iconfont.js
• [low prio] media:dataurls: Data URIs: https://github.com/unic/estatico/blob/develop/gulp/media/dataurls.js
• media:imageversions: https://github.com/unic/estatico/blob/develop/gulp/media/imageversions.js
• media:copy: Make more generic (not bound to media files): https://github.com/unic/estatico/blob/develop/gulp/media/copy.js
• scaffold: Evaluate https://github.com/amwmedia/plop and Yeoman, e.g.
• serve: Based on browsersync
• watch (with proper dependency graph)
• document: Replace with proper READMEs for every task

Examples:

• See existing packages

Criteria:

• Exporting plugin factory
  • Taking options overwriting defaults as argument. Defaults for a "standard" gulp task:
    • src: Which files to read
    • srcBase: See docs
    • dest: Where to write to
    • watch: Which files should trigger a rebuild
    • plugins: Passed to specific gulp plugins
    • logger: Used for infos, debugging and errors. Most probably an instance of estatico-utils's Logger
  • Exposing options for everything configurable. Setting plugin options to null should disable the corresponding step
  • Returning a gulp stream in case of "standard" gulp tasks. Most probably returning a promise otherwise since we don't want to deal with callbacks.
  • Using gulp-plumber to prevent stream from unpiping, adding .on('error', err => config.logger.error(err, env.dev)) to every step possibly throwing.
  • Using joi to validate options: https://github.com/unic/estatico-nou/blob/master/packages/estatico-handlebars/index.js#L9-L37
• Using gulp 4 (if using gulp). Probably not having an effect on "standard gulp tasks since they mostly use gulp.src and gulp.dest which should not break anything for us
• Not using gulp-util which will be deprecated. Rather rely on the underlying modules (plugin-error, fancy-log, chalk, minimist etc.
• Documenting usage and options in README.md
• Adding tests