Add a parameter to not clean temporary directory

[dploeger]

When a run of octocatalog-diff fails, it would be great if it wouldn't clean the temporary directory so that additional debugging can take place.

As a workaround, I currently comment out

octocatalog-diff/lib/octocatalog-diff/util/util.rb

Line 73 in 5071b97

at_exit { remove_temp_dir(the_dir) }

currently, but a parameter like "--keep-temporary-on-fail" would help here.

[claviola]

You can do this by setting OCTOCATALOG_DIFF_TEMPDIR. See "advanced environment variables" in doc/advanced-environment-variables.md.