postgres: Feature request: Support creating SQL fragments using tagged literal

[jonasb]

I've opened a PR (#395) to add another tagged literal, called fragment. When you create a fragment it can be used in a sql tagged literal.

Example usage:

const sharedValues = fragment`${123}, ${'admin'}`;
await sql`INSERT INTO users (id, credits, role) VALUES (1, ${sharedValues}), (2, ${sharedValues})`;

const column = isFoo ? fragment`foo` : fragment`bar`;
const filter = fragment`${column} = ${123}`;
return sql`SELECT ${column} FROM table WHERE ${filter}`;