Consider removing dependency on argcomplete

[gnarlyquack]

Although I'm sympathetic to what the package is trying to achieve, I don't know that I like its solution, especially given the minimal benefit it's providing us:

• Global completion appears limited to Bash, and it looks like Gourmet may not even be doing what it needs to do to support it (i.e., have "PYTHON_ARGCOMPLETE_OK" somewhere in the code. I'm not even sure where we would need to put it given we're using a setuptools entry point).
• Non-global completion requires editing one's shell configuration, which is probably problematic if attempting to run Gourmet in a Python virtual environment, and leaves lingering configuration behind if Gourmet is uninstalled.
• It appears to work by actually executing the application up until the point argcomplete.autocomplete is called, at which point it "hooks" execution of the application and exits if it determines a completion is required. As the documentation mentions itself, this is subject to side effects and performance issues if not careful.
• Given Gourmet is a GUI app, I wouldn't expect any but a handful of people (mostly developers) to be executing Gourmet from the command line anyway.

If we really want this feature, I'm more inclined to ship a shell/Bash completion script that would exist external to Gourmet itself and could be installed to standard configuration directories.