Feat: customize reportDir

Hey guy, thanks for the great work of `mochawesome-report-generator`.

 I've found for `reportDir` option, it was assigned to `path.resolve(process.cwd(), reportDir, filename)`. But sometimes we need to specify `reportDir` to some other path which is not based on the current working directory of the Node.js process. 

so `path.resolve(process.cwd(), reportDir, filename)` is actually not 100% customized.I've made the PR to add a `useCwd` option which will enable developer to customized the `reportDir`(actually I need this for my project).

Thanks again and wait for your reply

src/lib/main.js

@@ -80,7 +80,7 @@ function getTimestampFormat(timestamp) {
       return 'isoDateTime';
     default:
       return timestamp;
-  }f
+  }
 }
 
 /**