Add pre_alloc configuration option

Add pre_alloc configuration option

The new option is used to pre-allocate pools of memory at program start.
This could be useful if your program need to load a starting working set
into memory before doing the actual work, as it might avoid some useless
initial collections while the program is just doing allocation but no
memory is yet freed.

The option takes a numeric parameter indicating the size of the initial
pool in MiB. It can optionally specify an arbitrary number of pools to
create by appending "x" and the numeric value of the number of pools. Each
pool will have the specified pool size.

For exmaple, pre_alloc=50 will allocate an initial pool of 50 MiB, while
pre_alloc=5x10 will allocate 10 pools with 5 MiB each. Anything that can't
be parsed correctly (like "", "5x", "5a10", "5x10x", etc.) make the
starts with no pre-allocated memory as usual.