options.txt is read with default OS encoding

The bug

It looks like the options.txt file is read and written using the default OS encoding. This can cause problems with resource packs whose name contains non-ASCII characters.

How to reproduce

1. Create a resource pack with a non-ASCII character in its name or download the attached §3MC-117449.zip and place it in the resource pack folder
2. Select the resource pack
3. Restart the game
   → The resource pack is not selected anymore. Additionally the options.txt file shows a malformed resource pack name ("§3MC-117449.zip")