Adds post_clone_commands attribute for each project

- The new attribute is optional
- The new attribute should point to an array of strings
- Each string will be treated as a command to run within the repo directory
- If the command does not succeed (returns non-zero) it is ignored and the next
  command is run

tracker:[#74278058] github:[#5]
Merge pull request #7 from pivotal-sprout/post_clone_hook-74278058

Adds post_clone_commands attribute for each project