Add ability to dynamicaly skip a prompt

[fujifish]

It is sometimes useful to control whether to display a prompt to the user based on previous answers.
This PR adds a new ask method to a property that enables to skip the prompt based on custom logic.

E.g.:

  var schema = {
    properties: {
      proxy: {
        description: 'Proxy url',
      },
      proxyCredentials: {
        description: 'Proxy credentials',
        ask: function() {
          // only ask for proxy credentials if a proxy was set
          return prompt.history('proxy').value > 0;
        }
      }
    }
  };

I've also included two other minor enhancements:

1. If prompt.message is set to an empty string then do not print the delimiter
2. If a property is not valid and has a message defined, then do not print the generic "Invalid input for ..." message.

Great project by the way!

[gangstead]

This functionality has been fixed in the created-before-this-but-only-recently-merged #61

[gangstead]

Sorry this sat for so long, I've started helping on this project (yesterday). This looks really good, thanks for including updates to the docs too.

I've put some other changes in which makes this need an update. Do you think you could do a pull and fix the conflicts so I can accept this PR?

[fujifish]

done both requests.

[gangstead]

I fixed the test that was failing. It was looking for "Invalid Input" but now that extra line doesn't show up if schema.message is set.

[gangstead]

Thanks!

[fujifish]

Thanks. When is a release to NPM expected?

[gangstead]

As son as i can get permission i will. Working on it with the admins #144

[jcrugzz]

v0.3.0 is relased in npm