Initial implementation of a minimal custom rules API (currently Visual Studio only)

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branch : feature/custom-rules
Added support for custom rule list variables (VS only)

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branch : feature/custom-rules
Modernizing pass over VS'10 file generation

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branch : feature/custom-rules
Add custom rules to VC 2010 filters

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branch : feature/custom-rules
Bug fixes and modernizing of VC 2010 exporter

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branch : feature/custom-rules
Added missing extension targets section for custom rules

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branch : feature/custom-rules
Add Premake executable directory to search path for the system script
Merge with default

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branch : feature/custom-rules
Merge basic support for custom Visual Studio rule files