Input prompt

[jb55]

Implement script_handle_input, which prompts on free variables.

For example, with a script such as:

OP_IF
    [HASHOP] <digest> OP_EQUALVERIFY OP_DUP OP_HASH160 <seller pubkey hash>            
OP_ELSE
    <num> [TIMEOUTOP] OP_DROP OP_DUP OP_HASH160 <buyer pubkey hash>
OP_ENDIF
OP_EQUALVERIFY
OP_CHECKSIG

it should prompt for input on <digest>, <seller pubkey hash> etc. otherwise, provide input via cli arguments. Maybe eventually we will have variables...

[jb55]

Not how well this would interact with stdin, might have to provide via cli args...