Provide ability to specify percentile groups beyond quartiles

[ryangreenberg]

I think st is great and I use it all the time. One thing that would make it more useful for me is the ability to be more selective about the percentile groups of interest. For example, instead of the 25th percentile, 50th percentile, and 75th percentile values in a dataset, I would like to get the 90th percentile and the 99th percentile.

One possible way of writing this:

st --N --percentile=90 --percentile 99 --percentile 99.9  mydata.log
N	p90	p99	p99.9
3968275	470	800	3500

Another more compact way might be:

st --N --p90 --p99 --p99.9  mydata.log