Specs:
Behavior: Letter e is replaced with "3" Input: leet Output: l33t
Behavior: Letter o is replaced with "0" Input: boo Output: b00
Behavior: Letter I is replaced with "1" Input: I like Output: 1 lik3
Behavior: Letter s is replaced with "z" Input: unless Output: unl3zz
Behavior: Letter s is replaced with "z" unless it is the first letter of the word Input: size Output: siz3