Create `bash_update` script

- Update README with how/when to update
- Update help file with new flag
- Create new bash_core builtin man page for `bash_install` &
  `bash_update` commands
- Update `bash_install` man page to link to new bash_core man page
- Create `bash_update` man page to link to new bash_core man page
- Update install script with new update flag

Issue: #59