text: Title: Display relative, rather than absolute, paths in DAG plot
  Body: - ploomber, version 0.8.4
  - MacOS Catalina
  
  When calling `ploomber plot`, the generated DAG displays absolute path names by default. Change the default behaviour to display pathnames relative to the directory in which ploomber was called.
  Benefits of this:
  -  For projects, ploomber will often be called from the root directory as monitored by git, so this is the natural folder to display paths in DAGs relative to
  - Absolute paths give details of users' computer configuration
  - Relative pathnames are less cluttered (with extraneous information)
  