Add no fast forward option

We have use cases that do not allow users fast forward the media(to ensure they watched every second), and I just added this option into mejs.

I make this pull request since I hope this option can benefit other developers to achieve this kind of use cases.

And I did a small change on "Builder.py" to fix the Unicode encoding problem on Windows.

I'm new to github, so if I did something wrong please point out.

Thank you a lot.