Added option to override charset of subtitles for unsupported charsets.

Turkish characters had problems since the [jschardet](https://github.com/aadsm/jschardet) didn't have support for ISO-8859-9. I had to workaround and make the charset of the track element the languages first encoding option if the language had the overrideCharset boolean set to true. This worked for me and now Turkish Characters work good.

Note that automatic charset detection doesn't always work, so it would be better if we had a chance to change charset encoding manually until we find the right one for the subtitle instead of adding a custom utf-8 subtitle. This may be edited in later i guess.

[Issue #576 ](https://github.com/popcorn-team/popcorn-app/issues/576), [Issue #594 ](https://github.com/popcorn-team/popcorn-app/issues/594)