Adds file check to filesSrc filter

Hello,

I was running into an issue with `grunt todo` and nested folders.

    $ grunt todo
    Running "todo:default_options" (todo) task

    webapp/app.js

      line 1  TODO  // TODO HI
    Warning: Unable to read "webapp/controllers" file (Error code: EISDIR). Use --force to continue.

    Aborted due to warnings.

Digging around, adding `grunt.file.isFile` fixed the issue.