log1mexp -- adding yet another function to the wish list

[argriffing]

log1mexp(x) = log(1 - exp(-x))
According to this CRAN note it should be evaluated using either log1p or expm1 depending on some threshold value of x. Maybe I will add this. Just an editorial comment, the name is a little funky because you might think it would compute log(1 - exp(x)).