add a flag to allow for a clean single word transcription

[hungLink]

Hi there! This feature would allow for a user to add the -s or --single flag and feed the script one word.

if they do, the output looks like this:

❯ ./eng_to_kana/eng_to_kana.py --single test
テスト

Instead of this:

❯ ./eng_to_kana/eng_to_kana.py test
['test']
[['テスト']]

This allows the tool to be better suited for piping and using in various bash-y toolchains.