fix iodide-project/iodide#1423: Don't crash if there is no Python code

Check packages against Pyodide ABI version

Modified ``Makefile`` and ``packages/Makefile`` to get ``PYODIDE_PACKAGE_ABI`` from ``Makefile.envs``
Made ``checkABI`` as a public API function.
Modified ``file_packager.py`` to check for the compatible ABI number.
Defined ABI number with which pyodide is built in Makefile.envs
Accepted ABI number in ``file_packager.py`` as an argument.
Modified ``buildpkg.py`` and ``buildall.py`` to account for the new argument.
Added the corresponding ``test_different_ABI`` test.

Update docs about using Pyodide from Iodide.

Fix preloading on Chrome

Upgrade emscripten to 1.38.22

Add style to matplotlib widgets

Add style to matplotlib widgets
Add Color Change on Hover Functionality
Apply System Fonts to buttons with text

Fix #291: Allow loading of arbitrary packages

Make bin/pyodide display help rather than crash

Convert Numpy arrays to TypedArrays when possible

Explicitly use python3 for pyodide_build

Add ``Humor Sans`` font for ``xkcd`` plots.