Replace sed with bash’s pattern substitution

Three of the four uses of sed in the script have been replaced with
bash’s pattern substitution (${parameter/pattern/string}), which should
speed up execution a bit since we’re no longer spawning three processes
per substituted parameter.

The one remaining use of sed doesn’t have anything to do with patterns:
sed is used to read the second line of the cheat file (the command). I
could replace this with a head/tail pair. pro: removes all dependencies
to sed; con: spawns two processes instead of one. We’ll see.
Use bash syntax to set IFS to \n
Fix multiple variables

Another rendition of "how did I never catch this bug before?"
In other news, when you’re already setting $IFS for your purposes, you
should think twice if you really need to quote something.

(What happened was that all lines in the "prompt" part were seen as one
line, which lead to interesting behavior: If you declared several
variables “a:desc a[\n]b:desc b”, the script would see a variable called
“a” with the prompt “desc b”, and discard the rest.)
Add two dd cheats

to rip and burn an iso image, respectively.
Fix prompts with multiple colons

Previously, if the variable declaration contained several colons
(“varname:Like this: Look!“), only the last part (“ Look!”) would be
used because the regex to remove the “varname:” part was too greedy.
Now, it doesn’t eat colons, and thus matches the whole prompt.