Allow access to Clients by ID or selection function

[sminez]

Similar to #15 , penrose should expose a way for users to get reference to specific Clients either by ID or Selector:

pub fn client(&self, selector: Selector<Client>) -> Option<&Client>;
pub fn client_mut(&mut self, selector: Selector<Client>) -> Option<&mut Client>;
pub fn clients_where(&self, selector: Selector<Client>) -> Vec<&Client>;
pub fn clients_where_mut(&mut self, selector: Selector<Client>) -> Vec<&mut Client>;