EESSI bot v0.6.0

This is a minor release of the EESSI build-and-deploy bot.

Improvements:

• move merged PR job directories to trash_bin_dir (#271)
  • the target directory can be defined with the app.cfg setting trash_bin_dir
  • it uses shutil.move which tries to use mv if source and target are on the
    same filesystem
• add setting to give all jobs a unique name (#273)
• move closed PR job directories to trash_bin_dir (#275)
• add filter for accelerators (#276)
• add support for updating Slurm options through user-defined python module in
  target PR (#277)
• use GitHub API for downloading the diff of a PR (#278)
• add documentation about private repos (#279)
• pass accelerator value to job scripts (via job.cfg) and extend PR comment if
  the accelerator argument is used (#280, #282)

New app.cfg settings (see README.md and app.cfg.example for details):

• (optional) allow_update_submit_opts in section [buildenv]
• (required) job_name in section [buildenv]
• (required) moved_job_dirs_comment in section [clean_up]
• (required) trash_bin_dir in section [clean_up]
• (required) with_accelerator in section [submitted_job_comments]