Extend "optional" flag for ConfZFileSource to ignore more exceptions

[ajanon]

See PR #50 and issue #52.

optional now makes it possible to ignore if a command line argument is not set (through file_from_cl) or if an environment variable is not present (file_from_env).

[silvanmelchior]

Thanks a lot for your PR!

One small thing: I wouldn't allow _get_format to fail, as this does probably not reflect the intention of the user using the optional flag.

[ajanon]

That makes sense!
It does look kinda ugly, though:

try:
    file_path = cls._get_filename(confz_source)
except ConfZFileException as e:
    if confz_source.optional:
        return
    raise e
file_format = cls._get_format(file_path, confz_source.format)
try:
    file_content = cls._read_file(file_path, file_format, confz_source.encoding)
except ConfZFileException as e:
    if confz_source.optional:
        return
    raise e

Do you think this could be written better? The order of the operations needs to stay the same, and I cannot see any other way to have get_format outside the block. I could also check inside each function, but as mentioned in the issue, this may not be more maintainable.
If this is fine with you, I will update this PR accordingly.

[silvanmelchior]

True, it looks a bit ugly, but I don't see a good alternative. A context manager which would catch and optionally ignore the error like with AllowException(ConfZFileException, enabled=optional): ... does not help as we would still need to change the control flow (no follow-up calls if the first one fails). So I think we can go with your proposed solution.

[silvanmelchior]

Great, thanks! I will merge and release a new version tmr.