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

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.

Closes #112

datasette/cli.py

@@ -116,10 +116,13 @@ def package(files, tag, metadata, extra_options, **extra_metadata):
 @click.option('--page_size', default=100, help='Page size - default is 100')
 @click.option('--max_returned_rows', default=1000, help='Max allowed rows to return at once - default is 1000. Set to 0 to disable check entirely.')
 @click.option('--sql_time_limit_ms', default=1000, help='Max time allowed for SQL queries in ms')
-@click.option('--load-extension', multiple=True, help="Path to a SQLite extension to load")
+@click.option(
+    'sqlite_extensions', '--load-extension', envvar='SQLITE_EXTENSIONS', multiple=True,
+    type=click.Path(exists=True, resolve_path=True), help='Path to a SQLite extension to load'
+)
 @click.option('--inspect-file', help='Path to JSON file created using "datasette build"')
 @click.option('-m', '--metadata', type=click.File(mode='r'), help='Path to JSON file containing license/source metadata')
-def serve(files, host, port, debug, reload, cors, page_size, max_returned_rows, sql_time_limit_ms, load_extension, inspect_file, metadata):
+def serve(files, host, port, debug, reload, cors, page_size, max_returned_rows, sql_time_limit_ms, sqlite_extensions, inspect_file, metadata):
     """Serve up specified SQLite database files with a web UI"""
     if reload:
         import hupper
@@ -143,7 +146,7 @@ def serve(files, host, port, debug, reload, cors, page_size, max_returned_rows,
         sql_time_limit_ms=sql_time_limit_ms,
         inspect_data=inspect_data,
         metadata=metadata_data,
-        sqlite_extensions=load_extension,
+        sqlite_extensions=sqlite_extensions,
     )
     # Force initial hashing/table counting
     ds.inspect()