add run_shell_cmd hook

[boegel]

• 😎 : run_shell_cmd hook that triggers before/after running a shell command (works for both run_cmd and run_cmd_qa);
• ⚠️ some exceptions necessary for commands run in systemtools + set_tmpdir, because they are run before EasyBuild configuration is properly set up (so build_option doesn't work yet);
• 🔥 pre_run_shell_cmd_hook can modify the command before running it;
• 📖 additional context is available:
  • working directory;
  • output + exit code for post_run_shell_cmd_hook;
  • discriminate between interactive and non-interactive commands;
• ✅ tests have been added/extended to ensure implementation works as designed;

[jfgrimm]

lgtm

[jfgrimm]

Going in, thanks @boegel!