Fix for usage of --excel-add-features

Update excel.py

Fix --excel-add-feature

Fix for usage of --excel-add-features

Update excel.py

fix option feature

Introduction of option --excel-add-features

option --excel-add-features will generate an addition table in Excel spreadsheet listing all the Yang features declared in the input Yang modules

Fix for feature without prefix

Fix the error happening because when the if-feature statement lacks the module prefix, it tries to look it up in ctx.features[module.arg] but fails because the key doesn't exist.