Add ability to change sampling rate

Right now, the sampling rate from `xhprof_sample_enable` is hardcoded to once every 100ms. This isn't granular enough for much of the stuff I need it for, and definitely isn't granular enough to produce things like flamegraphs from.

It'd be nice to be able to set this via either an INI setting or via a parameter to `xhprof_sample_enable` to allow more detail in samples.
