Define equipment requirements

[ibboard]

Taken from ticket:27, since this requires a slightly different bit of coding (albeit based on the same ideas). Some equipment may need certain constraints. Ideas are:

• Unit must take item X to have equipment item Y (e.g. "must have Mark of Warrior God to take Axe of Warrior God")
• Equipment item X can only be taken Y times per army (e.g. "rare scroll can only be taken three times per army")
• Points limits on unit equipment (e.g. "up to 40pts spent on any combination of items Q, R, S...Y or Z")
• Equipment X can only be taken in armies of Y points and above (e.g. "Historic National Battle Standard can only be takin in armies of 3000pts or more")