Key format parsed incorrect if only a single argument is given

[astamminger]

Running zotero-bibtize with only a single argument in the --key-format option, i.e.

zotero-bibtize --key-format [author] ...

only the first character of the content, i.e. a, is parsed. This can be fixed if the argument passed with
quotation marks, i.e.

zotero-bibtize --key-format "[author]" ...

However, it works as expected (without quotation marks) for multiple arguments like [author][journal

[astamminger]

Obviously click does some magic when parsing the key from the command line an does not like the braces []. At least it works when the braces are correctly escaped, i.e. --key-format \[author\] would lead to the desired result.