Add ability to detect MCP feature states

[NullCascade]

I'd like to add this before 0.10 launches. The function should take a single argument, the ID of the feature, and return the state of the feature.

MCP keeps track of what features are enabled in ./mcpatch/installed. Starting at 0x3C in the file, it seems to contain a list of 32-bit integers. Each integer matches an ID in the describe.json. If the number is negative, the feature is disabled. If the number is positive, it is enabled.

Example usage:

long isFeatureEnabled
setx isFeatureEnabled to xIsCodePatchFeatureEnabled 14
ifx (isFeatureEnabled)
    MessageBox "User has feature #14 enabled. Item values increase with enchantment value."
endif