make WIFI_MANAGER_MAX_PARAMS customizable

[mtiutiu]

I think it's pretty useful to have this parameter customizable so that users can change it from outside if required (e.g platformio, make, etc).

For example when using platformio I can set this in platformio.ini:
build_flags = -DWIFI_MANAGER_MAX_PARAMS=20

This allows me to change the value at compile time without modifying the library source code. I also provided the default value for it when not defined outside or somewhere else - you can see that in the code changes that I want to propose.

This approach can be used for other defines in this library source code if desired/required.

Very nice work btw regarding this library - very useful. Thanks.

[tablatronix]

Note that setting this is no longer needed, as memory is autoallocated for params now

[mtiutiu]

I noticed that. So yeah it kinda makes no sense now.