Feature request: plugins.sql-support avoid quote the string on user demand.

If we are loading as values a set of table.columns values from a database like occurs in: https://github.com/mistic100/jQuery-QueryBuilder/issues/221, our values are string, but we want get it without quotes, because are columns not statistic values.

This is current impossible to handled in the getSql method because it just check for the value type:
https://github.com/mistic100/jQuery-QueryBuilder/blob/73cb350b57610d2632e1eda4bb52c27195a303af/src/plugins/sql-support/plugin.js#L321-L323

One idea is add a new sqlStatements that check for a variable in the current rule and with base on that disable the quote for that rule or not. Then the user can set that value in the rule on demand. But we can not add by our self an sqlStatements on demand. Also the usage of an sqlStatements in a standard sql have not semantic sense.

So, a solution, a workaround, an idea?

Start with src/plugins/sql-support/plugin.js, especially lines 321-323, and read the linked jQuery Query Builder issue for the motivating case. Clarify how a rule should distinguish column identifiers from literal strings and what SQL output is expected in each case; done means the user-facing behavior and implementation approach are agreed.