Feature: Process templates in preprocessors config

Hey Folks,

like many other people I use lodash templates in my Grunt configuration to manage directory names, filenames and other things like ``package.json`` information for reuse.

Example:

    grunt.initConfig({
        dir: {
            lib: 'lib',
            test: 'test',
            unit: '<%= dir.test %>/unit'
        },
        jsFiles: [
            '<%= dir.lib %>/**/*.js',
            '<%= dir.unit %>/**/*.js',
            'Gruntfile.js',
            'index.js'
        ],
        eslint: {
            all: '<%= jsFiles %>'
        }
    });

In **grunt-karma** it's possible for the ``files``and ``configFile`` configuration, but not for ``preprocessors``, which is an object with keys of files:

    preprocessors: {
        '<%= dir.unit %>/**/*.js': [
            'browserify'
        ]
    }

It would be nice if this gets merged.