Add function to set up / update the commit hooks

[rmbielby]

I'm thinking this could consist of:

• an initialiser that creates:
  • a .git/hooks/pre-commit script that loads dfeshiny and then runs a single dfeshiny function
  • datafiles_log.csv
• a single function, e.g. commit_hooks() that itself runs individual hook functions:
  • data_checker() - checks for unpublished data
  • style_code() - runs styler::style_dir()
  • check_analytics_key() - check for the presence of the analytics key