Add key argument to widgets to prevent DuplicateWidgetID (#7)

[andfanilo]

Add a key parameter to visualize methods so they can be called multiple times from the same script as long as the user provides a key parameter.

import spacy_streamlit

models = ["en_core_web_sm", "en_core_web_md"]
text1 = "Sundar Pichai is the CEO of Google."
text2 = "Randy Zwitch is Head of Developer Relations at Streamlit"
spacy_streamlit.visualize(models, text1, key="1")
spacy_streamlit.visualize(models, text2, key="2")

Fixes #7

[ines]

Thanks a lot, this is very useful 👍