Custom poll answers were previously printed into the gnuplot control
file directly, between double-quotes. As a result, if a poll answer
contains a double-quote, it could cause a syntax error in the gnuplot
script, or worse.

Gnuplot accepts standard C-style double-quoted string escapes, so I can
just use ruby's "inspect" method to generate a safetly escaped string.

Note that within the string, % still has to be escaped separately
(doubled) to avoid issues with the string formatting. As well, I have
disabled "enhanced" mode which allows using special characters for
formatting commands.

Fixes #3039