framework for different caching strategies

[springmeyer]

Driven by https://github.com/mapbox/tilemill/issues/922 we should think about how to make the caching (and cache flushing) functionality more modular.

So far for the experimentation in the 'live-cache' branch of tilemill this is all that is needed to trigger rec-caching upon save, but we can do better:

diff --git a/lib/millstone.js b/lib/millstone.js
index ef4beb0..fdd70b5 100644
--- a/lib/millstone.js
+++ b/lib/millstone.js
@@ -312,7 +312,7 @@ function resolve(options, callback) {
             if (uri.protocol) {
                 var filepath = path.join(cache, cachepath(l.Datasource.file));
                 path.exists(filepath, function(exists) {
-                    if (exists) {
+                    if (exists && !(l.cache_method && l.cache_method === 'live')) {
                         symlink(filepath);
                     } else {
                         utils.mkdirP(path.dirname(filepath), 0755, function(err) {