Use argparse sub-commands for functionalities

[jrsmith3]

The refmanage application will have many functionalities; I've listed the "test" functionality in issue #19, but there will be others. For example: replace all entries' BibTeX keys with the entry's DOI in a specified .bib file.

These different functionalities are very different and will require different sets of arguments. Therefore, it makes sense to use argparse sub-commands to implement the different functionalities.