Supports underscores in variable names

[KentonWhite]

Fixes issues #301 Adds config var underscore_variables If TRUE (default) will use underscores in var names When migrating an old project, defaults to FALSE

Added underscore_variables config

Skip cleaning underscore if underscore_variables = TRUE

Bumped version to 0.9.2

Types of change

• [ ] Bugfix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Other (documentation etc)

Pull request checklist

• [x] Add functionality
• [x] Add tests
• [x] Update documentation in man
• [x] Update website documentation

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Proposed changes