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"