Cannot use ':' value in `-m` option with `gsutil notifications` command

Custom attribute values cannot contain `:` character in `-m` option with `gsutil notifications`.
We can't escape, so can't specify the time like this:
`gsutil notifications create -m TIME:"12:00" ...`

The key-value of the custom attribute is simply split by `:` in `notification.py`, which raises an exception.
```notification.py
elif o == '-m':
if ':' not in a:
raise CommandException(
'Custom attributes specified with -m should be of the form '
'key:value')
key, value = a.split(':')
custom_attributes[key] = value
```

Start in notification.py at the -m option parsing shown in the issue. Check how custom attribute values containing colons should be accepted while the key:value requirement remains enforced; done means the example with TIME:"12:00" is parsed successfully without the existing validation being lost.

Written by the indexing model from the issue text.