Consider adding "minification" operations

[vmadman]

I am thinking about allowing 2 different ways for users to specify that one or more files should be minified...

Option 1: Config

{
    "uglify": {
        "js/thefile.js": [
            "js/something.js",
            "js/other/**.js"
        ]
    }
}

Option 2: Placeholder Files

Since I generally dislike the idea of requiring users to create a config file, I was thinking it could be interesting to allow users to create placeholder files that provided blobs in a similar way that the config refers to above.

For example, if the user created /src/content/js/thefile.js_ with the following contents:

something.js
./other/**.js

.. so, basically, the .js_ extension would be treated as being special. The same could be used for other extensions, such as .css_.

[vmadman]

This issue is, more or less, duplicated by #86. However, since it's not an exact duplicate, I am going to leave them both open, for now.