Allow --load-extension to be set via environment variables

[simonw]

This will make it easier to package up datasette in a Docker container with a bunch of pre-compiled extensions without the user having to remember to include all of the options every time.

Click has a mechanism for this: http://click.pocoo.org/5/options/#multiple-values-from-environment-values

[simonw]

I tesed this by first building and running a container using the new
Dockerfile from #114:

docker build .
docker run -it -p 8001:8001 6c9ca7e29181 /bin/sh

Then I ran this inside the container itself:

apt update && apt-get install wget -y \
    && wget http://www.gaia-gis.it/spatialite-2.3.1/test-2.3.sqlite.gz \
    && gunzip test-2.3.sqlite.gz \
    && mv test-2.3.sqlite test23.sqlite \
    && datasette -h 0.0.0.0 test23.sqlite

I visited this URL to confirm I got an error due to spatialite not being
loaded:

http://localhost:8001/test23-c88bc35?sql=select+ST_AsText%28Geometry%29+from+HighWays+limit+1

Then I checked that loading it with --load-extension worked correctly:

datasette -h 0.0.0.0 test23.sqlite \
    --load-extension=/usr/lib/x86_64-linux-gnu/mod_spatialite.so

Then, finally, I tested it with the new environment variable option:

SQLITE_EXTENSIONS=/usr/lib/x86_64-linux-gnu/mod_spatialite.so \
    datasette -h 0.0.0.0 test23.sqlite

Running it with an invalid environment variable option shows an error:

$ SQLITE_EXTENSIONS=/usr/lib/x86_64-linux-gnu/blah.so datasette \
    -h 0.0.0.0 test23.sqlite
Usage: datasette -h [OPTIONS] [FILES]...

Error: Invalid value for "--load-extension": Path "/usr/lib/x86_64-linux-gnu/blah.so" does not exist.