fix(sourcemaps): use absolute URL for sourceMappingURL

This fixes and issue where relative URLS would be required and
consequently not found. For example, a page at foo/bar would require
sourcemapss at foo/sourcemaps/index.js.map. This fix addresses it in a
way that all sourceMappingURLs are set to absolute, always requiring
from root /sourcemaps

lib/sourcemaps.js

@@ -72,7 +72,7 @@ function generateSourceMap(fileHref, sources, files) {
 
 function addSourceMappingURLToSource(file, source) {
   const sourcemapFlag = '//# sourceMappingURL=';
-  let ref = `\n${sourcemapFlag}${file.vname}\n`;
+  let ref = `\n${sourcemapFlag}/${file.vname}\n`;
   if (source.ext === 'html') {
     ref = `<script>${ref}<\/script>`;
     const marker = /<\/(body|html)>/;