use more bash features

[grondilu]

Cool project, and a nice way to exercice in bash scripting, while at the same time doing something potentially very useful.

I forked it here : https://github.com/grondilu/fla.sh

Still working on it, but I've published a few commits already to show where this is going.

I will likely rewrite it extensively, so I'm probably not going to request a merge, as it will be quite different. Still, I think some ideas already worth pointing out are :

• using $'...' quoting construct
• using here documents
• using mapfile to read a whole file into an array
• using RANDOM to pick a random element
• creating a center function to center text
• use fmt to format lines
• use pushd/popd mechanism when changing directories
• hiding cursor with ANSI escape codes
• overwriting a line with ANSI escape codes (instead of redrawing the whole screen)
• using parameter expansions, e.g. ${DECK##*/}
• using letters H, D, N, M, E instead of numbers for difficulty levels (possibly a matter of taste)
• declare variables, either with declare or local, especially for integer ones (with the -i option)
• use as few global variables as possible
• ideally the whole program should be a bash function, not a bash program
• separate the tasks of deck selection and quiz submission
• use bash's specific ((...)) and [[...]] constructs
• use return instead of exit when possible
• use ${COLUMNS:=$(tput cols)} and ${ROWS:=$(tput rows)} to fill the screen