/*
Given an input list of strings, for each letter appearing anywhere
in the list, find the other letter(s) that appear in the most
number of words with that letter.
Example:
['abc', 'bcd', 'cde'] =>
{
a: [b, c], # b appears in 1 word with a, c appears in 1 word with a
b: [c], # c appears in 2 words with b, a and d each appear in only 1 word with b
c: [b, d], # b appears in 2 words with c, d appears in 2 words with c. But a and e each
appear in only 1 word with c.
d: [c], # c appears in 2 words with d. But b and e each appear in only 1 word with d
e: [c, d], # c appears in 1 word with e, d appears in 1 word with e
}
*/