feat: configurable $templateCache key

````javascript
karma.configure({
  ngHtml2JsPreprocessor: {
    cacheIdFromPath: function(filepath) {
      return '/prefix/' + filepath;
    }
  }
});
````

Closes #7

lib/html2js.js

@@ -9,19 +9,26 @@ var escapeContent = function(content) {
   return content.replace(/'/g, '\\\'').replace(/\r?\n/g, '\\n\' +\n    \'');
 };
 
-var createHtml2JsPreprocessor = function(logger, basePath) {
+var createHtml2JsPreprocessor = function(logger, basePath, config) {
+  config = typeof config === 'object' ? config : {};
+
   var log = logger.create('preprocessor.html2js');
+  var stripPrefix = new RegExp('^' + (config.stripPrefix || ''));
+  var prependPrefix = config.prependPrefix || '';
+  var cacheIdFromPath = config && config.cacheIdFromPath || function(filepath) {
+    return prependPrefix + filepath.replace(stripPrefix, '');
+  };
 
   return function(content, file, done) {
     log.debug('Processing "%s".', file.originalPath);
 
-    var htmlPath = file.originalPath.replace(basePath + '/', '');
+    var htmlPath = cacheIdFromPath(file.originalPath.replace(basePath + '/', ''));
 
     file.path = file.path + '.js';
     done(util.format(TEMPLATE, htmlPath, htmlPath, escapeContent(content)));
   };
 };
 
-createHtml2JsPreprocessor.$inject = ['logger', 'config.basePath'];
+createHtml2JsPreprocessor.$inject = ['logger', 'config.basePath', 'config.ngHtml2JsPreprocessor'];
 
 module.exports = createHtml2JsPreprocessor;