Closes #1784

The --load-extension flag can now accept an optional "entrypoint" value, to specify which entrypoint SQLite should load from the given extension.

```bash

would load default entrypoint like before

datasette data.db --load-extension ext

loads the extensions with the "sqlite3_foo_init" entrpoint

datasette data.db --load-extension ext:sqlite3_foo_init

loads the extensions with the "sqlite3_bar_init" entrpoint

datasette data.db --load-extension ext:sqlite3_bar_init ```

For testing, I added a small SQLite extension in C at tests/ext.c. If compiled, then pytest will run the unit tests in test_load_extensions.pyto verify that Datasette loads in extensions correctly (and loads the correct entrypoints). Compiling the extension requires a C compiler, I compiled it on my Mac with:

gcc ext.c -I path/to/sqlite -fPIC -shared -o ext.dylib

Where path/to/sqlite is a directory that contains the SQLite amalgamation header files.

Re documentation: I added a bit to the help text for --load-extension (which I believe should auto-add to documentation?), and the existing extension documentation is spatialite specific. Let me know if a new extensions documentation page would be helpful!