Does not pass interrupts to running script

[jakeheis]

A fix was previously attempted in #20, but stalled by SwiftShell deficiency. It's now possible with SwiftShell (kareman/SwiftShell#61) or SwiftCLI. I'm happy to work on whichever implementation you'd prefer -- SwiftShell would obviously involve less changes, but I know you mentioned at one point a desire to remove the dependency on SwiftShell if possible.