Update validation regex and error message for module name question

[ardrigh]

PDK does not correctly validate module names if a module name is not supplied to pdk new module.

The user question prompts for a module name but fails to pass underscores as valid.

• pdk new module test_name will pass
• pdk new module will fail on test_name given as the module name.

This update uses the regex referred to in the docs for validating module names.

I have included a similar error message to the options validation line, while referring to "numbers" as the docs do.

[coveralls]

Coverage remained the same at 92.071% when pulling d471a2c on ardrigh:module_validation into 85c33fa on puppetlabs:master.

[bmjen]

Great find @ardrigh and thanks for the contribution!