add $if expression

dist/lib/run/filter.js

@@ -35,7 +35,29 @@ Filter = (function() {
   };
 
   Filter.prototype.canProcess = function(asset) {
-    var filename;
+    var comparison, filename, prop, truthy, value, _ref;
+    if (this.processOptions.$if) {
+      truthy = true;
+      _ref = this.processOptions.$if;
+      for (prop in _ref) {
+        comparison = _ref[prop];
+        value = asset[prop];
+        if (_.isString(comparison)) {
+          truthy && (truthy = asset[prop] === comparison);
+        } else if (val instanceof RegExp) {
+          truthy && (truthy = comparison.test(value));
+        } else if (typeof val === "boolean") {
+          truthy && (truthy = val);
+        } else {
+          this.log.warn("Unrecognized $if asset property: '" + prop + "'");
+          truthy = false;
+        }
+        if (!truthy) {
+          return false;
+        }
+      }
+      return truthy;
+    }
     if (this.extnames.indexOf("*") >= 0) {
       return true;
     }

src/lib/run/filter.coffee

@@ -31,6 +31,27 @@ class Filter
   # a filter unless it can handle the asset's text.
   #
   canProcess: (asset) ->
+
+    # Simple if expression on the filter
+    #
+    # f.addHeader(text: "...", $if: {extname: ".txt"})
+    if @processOptions.$if
+      truthy = true
+      for prop, comparison of @processOptions.$if
+        value = asset[prop]
+        if _.isString(comparison)
+          truthy &&= asset[prop] == comparison
+        else if val instanceof RegExp
+          truthy &&= comparison.test(value)
+        else if typeof val == "boolean"
+          truthy &&= val
+        else
+          @log.warn "Unrecognized $if asset property: '#{prop}'"
+          truthy = false
+        return false unless truthy
+      return truthy
+
+
     return true if @extnames.indexOf("*") >= 0
 
     # must hanlde cases like ".coffee.md"