Allow multiple -a args for Stetl main prog

[justb4]

Currently only one -a argument can be passed to set either a list of options or a single options (.args) file. Allowing multiple -a arguments allows for more simpler overriding of for example default options. For example:

stetl -c my.cfg -a default.args -a my.args

or

stetl -c my.cfg -a default.args -a db_host=host -a db_user=me -a db_password=xyz

This allows to keep all default args and in my.args or explicit settings just have a few options like passwords.
The order of the -a args will determine overriding order. Args passed via the Environment like stetl_db_password will still prevail over any commandline args.

[justb4]

This has been fixed and can be closed. Supports the following issue:
nlextract/NLExtract#249 and Docker deployments within SE:
https://github.com/smartemission/smartemission