Add support for more fractional values

[Ian Lewis]

Currently WikiExtras supports fractional notation for 1/2, x/4 and maybe a few other values.

This works pretty nicely if all you want to show are these particular fractions, but it looks pretty bad when you place something like 1/16" next to 1/4".

It was this kind of issue that made me notice #13916.

It would be nice if the fractional handling were generic and could handle any fraction. However, that may be impractical. If so, then handle at least denominator powers of two through 1/64 (8, 16, 32, 64). For English measurements, fractions down to x/64" are common.