http://www.voupr.com/plugin.php?name=taplockturbo

Works like +turbo (press to turbo, release to stop turboing), unless you double-tap it. When double-tapped, it locks turbo until the next time you press it.

For more details, read the voupr page or use /taplockturbo with no arguments.

Note: This plugin is smart enough to unlock turbo when you jump or undock. It is not smart enough to unlock when you run the ship out of energy or disengage the turbo through some other means.