Add an example singleton object, FileMatcher.

It will be used to illustrate the power of higher-order functions.
Add a generator for files containing a query string.

Because, you know, sometimes you don't know what exactly your
files end in, so filesContaining lets you search in any position.
Some duplication is beginning to creep in...
Add a generator for files matching a regex.

Oh boy, the duplication is piling up now!