Write a function words_beginning_with(wordlist, head) that behaves as follows.
The argument wordlist is a list of strings, while the argument head is
a string. words_beginning_with(wordlist, head) returns a list consisting
of those strings in wordlist that begin with head. If none of the
strings in wordlist begin with head, it returns the empty list.
The order of list elements in the list returned by the function should match the order
of their occurrence in wordlist.
