The match pattern is now configurable

The match pattern is now configurable, as is the decision of whether discard the non-filename parts of the pattern or not.

For backward compatibility, I have set the default pattern to:

    /DIGEST\(([^\)]*)\)/g

…though I argue the default should simply match items in single quotes, double quotes, and within the parentheses of `url(…)`:

    /(?:['"]|url\(\s*['"]?)([^'"\)\s]+)(?:['"]|['"]?\s*\))/g

That would, of course, be a breaking change.