Allow words unknown to aspell

Allowed words are listed in files named using the following scheme:
	codespell_lib/tests/data/en-additionnal.wordlist
	codespell_lib/tests/data/en_GB-additionnal.wordlist
	codespell_lib/tests/data/en_US-additionnal.wordlist

Prefixes "en", "en_GB" or "en_US" match one of the supported languages.

We cannot add words with dashes such as "low-colour" or "high-colour"
to these lists, aspell doesn't allow that.

Since most if not all these words do exist in aspell dictionnaries,
I believe we are merely working around an aspell bug.