Add ability to switch and/or add npm tools

[paceaux]

Please change the task runner so that it points to a folder containing tasks, rather than actually having the tasks written in it. This adds flexibility and choice for which NPM tools can be used with DXA:

• create a directory called grunt/tasks
• configure the Gruntfile.js to pull individual tasks from this directory (see http://www.thomasboyt.com/2013/09/01/maintainable-grunt.html) for details

This could also be done by switching to gulp and using this in the gulpfile.js:

    var requireDir = require('require-dir');
    requireDir('./gulp/tasks', { recurse: true });

Benefits to clients

• Choice of CSS preprocessor (Less is not always the first choice for clients)
• Ability to use Sass as a preprocessor (necessary if client chooses to use Bootstrap 4)
• Ability to add a JavaScript lint utility, such as ESLint or JSHint
• Ability to add a CSS utility, such as Autoprefixer or PostCSS

Especially with Bootstrap 4 switching to Sass, and that the Bootstrap community is considering a postCSS port, it would be very useful to, at minimum, be able to switch the CSS preprocessor from Less to Sass.

I'll be able to provide a Sass version of DXA in the next few days, if you wish.

[rpannekoek]

TSI-1373 (internal issue ID for tracking purposes)