Provide a 'true' password prompt by hiding the amount of characters entered

[hierynomus]

By default when only providing a Prompt.Mask character, it is impossible to hide the amount of characters entered as you cannot enter either an '' (empty rune), or the 0 (null terminator).

The most backwards compatible way to fix this is by introducing a new Prompt.Password bool field that when set, defaults the Prompt.Mask to the 0, allowing promptui to completely hide the text entered, as is common in password prompts on the terminal.

[CLAassistant]

All committers have signed the CLA.

[hierynomus]

Any chance of getting this included in a next release? Anything that needs working on?

[szh]

It looks like this behavior can be achieved by setting prompt.Mask to a space ( ) character. See #133

[hierynomus]

Correct, never noticed that!. Thanks