Homework 3
Assigned 20 Oct; due 8 Nov.

Write a C program named "cipher" to implement a substitution cipher,
replacing certain letters of the alphabet (and possibly numbers,
punctuation markers, etc.) by other letters (numbers, etc.)

The program has one required parameter and two possible options
"-e" ("encode") and "-d" ("decode"); only one of these two may appear,
and if it does, it must appear before the parameter.  The parameter
is the name of a text file containing exactly two lines of the same
length.  (If it doesn't contain exactly two lines, or if they aren't the
same length, your program may print an error message or whatever you
like.)  These two lines represent the substitution cipher: a character
on the top line will be replaced by the character directly beneath it on
the bottom line.

After reading in this file, the program will act as a filter: for each
character it reads from stdin, it will find that character in the top
line and output the corresponding character in the bottom line.  If a
character appears twice on the top line, you may use whichever one you
wish; if it doesn't appear at all on the top line, output it without
changing anything.

For example, if there were a file named "Caesar" containing the two lines

ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz
DEFGHIJKLMNOPQRSTUVWXYZABCdefghijklmnopqrstuvwxyzabc

and I typed "cipher Caesar", the program would read from stdin:
whenever it read an A, it would output a D; whenever it read a B, it
would output an E, and so on; whenever it read a z, it would output a c.

Similarly, if I had a file named "upcase" containing the two lines

abcdefghijklmnopqrstuvwxyz
ABCDEFGHIJKLMNOPQRSTUVWXYZ

the program "cipher upcase" would effectively translate every lower-case
letter it read into a capital (upper-case) letter.