Added prefer_local_executable: true option so that users are able to install chrome-headless-render-pdf and puppeteer locally

[ryanzidago]

For instance, in the context of a Phoenix application, users can now chose if they would rather install chrome-headless-render-pdf and puppeteer within the /assets/node_modules directory.

• Having npm packages installed locally makes it easier to deploy an application to Heroku, where it is unrecommended to having them installed globally.

• Furthermore, with this prefer_local_executable, users are not required to do complex manipulation with Makefile, granted that they do not use mix release.

• The README.md was adapted to include the options, typos were fixed.

• A test was written to check if the option works as intented.

[gutschilla]

Hu @ryanzidago ,
thanks for the idea and implementation. I have have merged your code here as a result:
#82