Allow config file path be set by an environment variable

[enchuanhuang]

At our facility, we have multiple users, therefore, different home directories, using the same badger software. I suggest to check an environment variable like BADGER_CONFIG_PATH. If the env variable is empty, then use the default options. If it is not empty, use the environment variable as the config file path.

Please assign it to me as we have this implemented.

[nstelter-slac]

so if the new env variable is set, but then badger is ran with the -cf cmdline arg, would the cmdline arg value be used over the env var value?

  -cf CONFIG_FILEPATH, --config_filepath CONFIG_FILEPATH
                        Path to the config file

also, would "displaying the curr config file in gui somewhere" be nice to have for your use-case? (#218). i've been wanting this myself