Add OpenOptions to tokio-fs (#390)

Add an `OpenOptions` struct to `tokio-fs` that mirrors the one found in
`std`. Also provide a conversion from a `std` instance to a Tokio instance.

src/fs.rs

@@ -10,4 +10,5 @@
 pub use tokio_fs::{
     file,
     File,
+    OpenOptions, 
 };