[CR] merge special_freq and special_attack in JSON

If we're going for multiple special attacks (multiple defences next?) it would be nice to have sane JSON format for them.

This merges special_freq and special_attack into special_attacks with format: `special_attacks: [["BITE", 3], ["DOOM", 10], ...]`.

Questions/comments are welcome... If we agree on this I'd also merge special_when_hit and special_when_hit_freq into an array (or array of arrays, if we're going for multiple...).

Last note: monsters.json has been pretty-printed by the parser I use, if this is 'wrong style' please provide some tool that will format it 'properly'.
This in particular is a style regression.