Given a string, replace every letter with its position in the alphabet.
If anything in the text isn't a letter, ignore it and don't include it in the output.
"a" = 1, "b" = 2, etc.
alphabet_position("The sunset sets at twelve o' clock.")
-> "20 8 5 19 21 14 19 5 20 19 5 20 19 1 20 20 23 5 12 22 5 15 3 12 15 3 11"
- The output should be a string of space separated numbers.