Better column name handling for files

This release changes how columns (properties) are named when syncing files, and introduces a clean_colnames config.

Breaking change

There was a bug in the previous automatic column renaming logic, that inadvertently removed underscores. This release introduces a clean_colnames config for files that defaults to false and will not alter the column name, but when set to true it will convert column names to snake_case (a best effort, at least). The metadata column "LastModifiedDate" will not be converted to snake_case.