Make memory limit configurable

[getdatakick]

The sitemap generation contains memory consumption logic.

When PHP use more than 100MB of data during sitemap generation, data are flushed into sitemap file, and process is prematurely interrupted -- response contains redirect url to continue process.

This can be useful feature for hostings with limited resources, but totally unnecessary when server has enough memory.

Module configuration page should contain settings to control this behaviour -- change the memory threshold, or even disable this completely.