Corrected displayTransactionOptions method, created global hash

display transaction options method:
 1) updated this method to access the new global hash
 2) displays retail price (with 2 decimal places), with name of item
 3) puts a list of selection options for the user

global hash
 1) creating global hash made the code more DRY
 2) only need to call the read_file method once, instead of recreating
    the has everytime needed to use the hash