Added color_string function to highlight current word, using the current
position in the sentence (kept track of by col in parse_file). A
modification to print_box had to be made to lengthen the box where
formatting characters existed, as the are not displayed but take up
string length. While doing this I noticed an error in the replacing of
substrings, which resulted in an earlier word getting replaced rather
than the current. This was solved by only replacing the first occurance
from the current position (as noted by col).