allow importing as a script without GUI requirements installed

[terencehonles]

This change updates the script initialization to not fail on import errors, to finish without checking if --no-gui or --external-auth has been passed in the command-line, and instead raises an error when the application is initializing if running in GUI mode and the requirements are not met.

Minor changes also included:

• The --no-gui option is now stored as gui and defaults to True which changes comparisons to not be double negatives but retains the existing behavior.
• The App instance is allowed a single argument which is the command-line to parse, which allows starting the app with arguments from an import.

[terencehonles]

This will also allow packaging without all the requirements and this check https://github.com/simonrob/email-oauth2-proxy/pull/199/files#diff-8068eda2feddbc61595ced559aa73f8502813cf5cb1190a21e111ab1e8d19e88R71

[simonrob]

One benefit of the previous approach was that failed imports were more visible because they printed an error. With this new approach it's a lot harder to see which import is missing (and with #199 merged there won't necessarily be an easily-available requirements.txt to fix this).

At the moment, apart from for pystray and AppKit, is there any need to separate all of the imports and catch errors independently? If any single one fails and --no-gui isn't specified then we're going to exit anyway. So, perhaps a better approach is to bring back the original ArgumentParser, but use it to decide whether to display an error or not (i.e., if in GUI mode and imports fail).