Add support for callbacks

[brikis98]

To make boilerplate even more flexible and powerful, it would be great to support "callbacks" where you can provide arbitrary code to execute at specific times. For example, there could be a start callback that gets called just after Boilerplate gathers all variables but before it starts generating; a file callback called for each file and folder Boilerplate processes; and an end callback called after project generation is finished. The callbacks could be defined as simple shell commands in boilerplate.yml:

variables:
  - name: foo
  - name: bar

callbacks:
  start: echo "Boilerplate is generating files to $2 from templates in $1"
  file: echo "Boilerplate is processing file $1"
  end: echo "Boilerplate finished generating files in $2 from templates in $1"

[josh-padnick]

Can you give an example use case of this?

[brikis98]

Sure. Let's say you're using boilerplate to generate a skeleton for a react.js project. Actually, here's an example of just such a template: https://github.com/Granze/react-starterify

You could package that template as a boilerplate template and use the callbacks as follows:

• start: Look on GitHub and see if there is a newer version of the template available. If there is, prompt the user to download the latest version.
• file: when the package.json file is being processed, run the npm update command to fetch the latest version of each Node.js package (this may be a way to solve #8, BTW).
• end: run npm install to install all those dependencies and npm run watch to immediately put you into live reload development mode as soon as the project is finished generating.