near-zero and near-100 rounding option

[ianstormtaylor]

Right now using a simple percent format like:

const f = format('.1%')

Gives misleading results near 0 and 100:

f(0.0001) // 0.0%
f(0.9999) // 100.0%

Even though these are technically correct from a rounding perspective, they lead to confusion for users because it reads as "absolute". Which is why it's common to instead refer these as:

<0.1%
>99.9%

It would be really nice if there was a flag for this style of percentage formatting.