Added headless mode functionality

[james-wallis]

I've added a headless mode functionality to the generator so that the user is able to use this generator without the prompts.
The user must supply a --headless option to the end of the yo nodeserver command. This will create a project using the given defaults much alike using the prompt without entering any values.

To create a custom project the user can do something like:
yo nodeserver --headless='{"name":"projectname","services":["watson conversation","cloudant"]}'

I've updated the readme to reflect the added feature as well as listing out which services will be valid to make using headless mode easier.

Testing: I've added three tests.

1. Checks that the project is being created correctly in headless mode.
2. Checks the swagger file name aspect.
3. Checks the services being added.
   IMO I don't think we need to add anymore testing as I have shown that the behaviour of headless mode is the same as using prompts.

Would recommend that you viewed the commits with ?w=1 added to the end of the url as my atom has removed a lot of whitespace.

[coveralls]

Coverage increased (+1.5%) to 85.075% when pulling 8fa4c09 on jamesemwallis:development into 12a8bf3 on ibm-developer:development.

[coveralls]

Coverage increased (+1.5%) to 85.075% when pulling 17f75e3 on jamesemwallis:development into 12a8bf3 on ibm-developer:development.