Add version to __version__

[arnaudmiribel]

It's time to add the __version__ attribute! Copied how streamlit does it.

How to test

After pip install -e . && ipython, run:

In [3]: import streamlit_extras

In [4]: streamlit_extras.__version__
Out[4]: '0.4.4'

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[arnaudmiribel]

Fix #105

[blackary]

🤘