Fix common base path calculation (#35)

A RegExp is being used to subtract out the base path common to "dirname"
and "from", but the "from" path was not being properly escaped for use
in a RegExp. See:

https://developer.mozilla.org/en/docs/Web/JavaScript/Guide/Regular_Expressions
This commit stops npm test from throwing an uncaught exception of type
"RangeError: Maximum call stack size exceeded".

index.js

@@ -330,7 +330,8 @@ function processCopy(result, from, dirname, urlMeta, to, options, decl) {
     }
     relativeAssetsPath = path.join(
       relativeAssetsPath,
-      dirname.replace(new RegExp(from + "[\/]\?"), ""),
+      dirname.replace(new RegExp(from.replace(/[.*+?^${}()|[\]\\]/g, "\\$&")
+                                 + "[\/]\?"), ""),
       path.dirname(urlMeta.value)
     )
     absoluteAssetsPath = path.resolve(to, relativeAssetsPath)