Flatten nested cell arrays

Linearize a cell array tree structure

C1 = flatten({{1 {2 3}} {4 5} 6})
C2 = flatten({{'a' {'b','c'}} {'d' 'e'} 'f'})

% Outputs:
C1 =
[1] [2] [3] [4] [5] [6]
C2 =
'a' 'b' 'c' 'd' 'e' 'f'