add nobundle and sourcesLocation argument

[AdrienSeon]

This pull adds the following arguments:

--sourcesLocation to bundle command so that you can direct the script to a specific folder where sources are located, not only under the root of the repo as shown in this exemple.
Can be useful for repo clarity when developing abstract sources

--nobundle to serve command so that it does not bundle again the files since it can not use arguments for bundle. The old all in one serve command can still be used like this:
"serve": "paperback bundle --sourceslocation=sources && paperback serve -p 8000 --nobundle"

Code can surely be improved, I'm not experienced with commands environment in node. Feel free to modify it to make it cleaner.