Added message when poor vision slows down reading

Depending upon vision impairments and the overall light level, it can
take up to 5x longer to read (unless the player is specifically
carrying a lightstrip, as unlike other dim light sources those
specifically let you read at full speed).

This doesn't change any of the behavior, but adds a warning in the
message log when you begin reading with a penalty in effect.

It feels reasonable to tell the player, as the character would
instantly know they're having trouble making out the words. The
warning itself is reasonably vague as to why ("It's difficult to see
fine details right now. Reading will take longer than usual."),
leaving it up to the player if they do anything about it.

I had originally considered making it do a warning with a Y/N when you
go to read a book with the penalty active, but decided if somebody is
intentionally reading in dim light there are already more than enough
Y/N steps to reading a book.

This way leaves the player free to make mistakes, but at least makes
it easier to realize that they might be making a mistake in the first
place.