Add support for Multiple targets.

[markfredchen]

The code change focused on change "usemin" task to accept multiple targets. Now it is possible to use 'usemin' task like this:

usemin: {
    app1: {
        src: ['dist/app1/index.html'],
        options: {
            type: 'html',
            assertDirs: ['dist/app1/images/{,*/}*.png']
        }
    },
    app2: {
        src: ['dist/app2/index.html'],
        options: {
            type: 'html',
            assertDirs: ['dist/app2/images/{,*/}*.png']
        }
    },
}

[brunowego]

👍

[stephanebachelier]

@markfredchen should go to README.

If type is not html/css/js -> If type is different from html/css/js

[micw]

Great patch, +1

[dustin-page]

👍