custom promisify implementation for readline

[a7ul]

Fixes #36

Custom promisify fixes the issue with readline breaking in node <= v15.7.0

This might be happening because readline's question function has a callback function. And instead of returning error in the first argument it has the answer itself ?

• Tests pass
• Appropriate changes to README are included in PR

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[a7ul]

@googlebot I signed it!

[a7ul]

Yep the support for promisification of readline question was only added in node v15.8.0
See here: nodejs/node@19981ed
Related pr here: nodejs/node#33676

So this patch solves it for all version before node v15.8.0 and after too.
I tested this in node v15.7.0 without this patch and the issue still pops up.