Adds 'required' option to 'prompt_for_input'

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cc: @mbland

This PR adds an extra argument to @go.prompt_for_input. If the function is called with this argument, eg @go.prompt_for_input 'variable' 'prompt' '' '' 'required' and the user supplies no value, ie presses only enter, the prompt will reappear until the user actually supplies something. If instead of required another value is supplied, the behavior is the already existing one.

The rational behind this PR is that as it stands now, using @go.prompt_for_input in a loop to force the user to actually submit sth, is exactly identical to using read directly. It saves nothing. With this PR, it only takes one line and you are sure that the user will provide some input. Moreover, @go.prompt_for_yes_or_no already loops until the user supplies something. This PR makes the two functions more "feature compatible".

mbland / go-script-bash

Adds 'required' option to 'prompt_for_input' #237