Support loading extensions

[talmeme]

Quick fix that I'm using: Add these lines after sqlite3_open() in main.c's sql_open() :

    status = sqlite3_enable_load_extension(conn, 1);
    if (status != SQLITE_OK) janet_panic(sqlite3_errmsg(conn));

This enables the SQL function load_extension().

Ideally, as per the security warning in https://sqlite.org/c3ref/enable_load_extension.html, the SQL function load_extension() should be disabled, and extension loading should only be done through these SQLite API functions:

• sqlite3_db_config()
• sqlite3_load_extension()