wip: functions median and percentile

Hello

I am sending a prototype implementation of functions median and
percentile. This implementation is very simple and I moved it to
contrib for this moment - it is more easy maintainable. Later I'll
move it to core.

These functions are relative simple, there are not barrier for
implementation own specific mutations of this functions - so I propose
move to core only basic and well known form of these to core.

postgres=# select median(v) from generate_series(1,10) g(v);
median
────────
5.5
(1 row)

Time: 1.475 ms
postgres=# select percentile(v,50) from generate_series(1,10) g(v);
percentile
────────────
5
(1 row)

Time: 0.626 ms

This implementation is based on tuplesort and the speed is relative
well - the result from 1000000 rows is less 1 sec.

Regards

Pavel Stehule