Add `validate_with` function to password prompt

[qt2]

Closes #150

Implement validation function in Password which would be useful for checking the strength of passwords. This PR includes:

• Adding validator property and validate_with method to Password
• Refactoring the password confirmation process in order to reduce complexity
• Adding validation process in interact_on method
• Minor update on an example code

[smbcloudXYZ]

Are you still working on this? I check the test it's failing

[qt2]

Are you still working on this? I check the test it's failing

Sorry for the late reply. The test failure was due to an incorrectly configured import in doc-test. I will fix it and commit it later. Thank you for pointing this out.

[qt2]

@pksunkara

In order for interact_on to function without mut, the validator needs Fn instead of FnMut, so a new PasswordValidator trait was added separately from the Validator for Input. Also replaced &String with &str where possible.

[pksunkara]

You don't need a password validator trait and the String -> str. I won't be merging them. Please keep it similar to Input except for the mut part.

[qt2]

@pksunkara

Hmm, I have no idea using V::Err in validate_with method. Simply adopting your change suggestion raised an error "associated type Err not found for V". Could you tell me some workaround on it, if possible?

[qt2]

If using E instead of V::Err is fine, then I'll revert the last commit.

[pksunkara]

I was mistaken, Input has a trait already. So, this trait is okay with me.

The only change I would request is to keep the original &String and not &str. The validate function should be dealing in &T where T is the output of the prompt, which is String in the password prompt.

[smbcloudXYZ]

Hey @pksunkara, how's the release process for this?

[pksunkara]

Done.