Add more methods

[AsafMah]

Added

/// Insert `item` after the cursor
pub fn insert_after(&mut self, item: T);
/// Insert `item` before the cursor
pub fn insert_before(&mut self, item: T);

/// Remove the current item. The new current item is the item following the
/// removed one.
pub fn remove_current(&mut self) -> Option<T>;

From the RFC, that were left as an exercise to the reader (I'm a reader).