message: Add seeking support for ggez::filesystem::File
diff: use std::env;
  use std::fmt;
  use std::io;
  +use std::io::SeekFrom;
  use std::path;
  use directories::ProjectDirs;
  @@ -91,6 +92,14 @@ impl io::Write for File {
  }
  }
  +impl io::Seek for File {
  + fn seek(&mut self, pos: SeekFrom) -> io::Result<u64> {
  + match *self {
  + File::VfsFile(ref mut f) => f.seek(pos),
  + }
  + }
  +}
  +
  impl Filesystem {
  /// Create a new `Filesystem` instance, using the given `id` and (on
  /// some platforms) the `author` as a portion of the user
  