ensure_build_metadata() function for metadata

This will be run at compile time - the goal is to generate a build-
metadata.json file with a bunch of useful facts about the databases that could
be expensive to generate at run-time.

Example metadata:

    {
        "flights": {
        "file": "flights.db",
        "tables": {
            "airlines": 6048,
            "airports": 8107,
            "routes": 67663
        },
        "hash": "07d1283e07786b1235bb7041ea445ae103d1571565580a29eab0203c555725fd"
    }

So far we have a sha256 hash of the database file itself, plus a row count for
each table.

Fixes #11

templates/index.html

@@ -6,6 +6,7 @@ td {
     padding: 2px 4px;
 }
 </style>
+<pre>{{ metadata }}</pre>
 <table>
     <tr>
         {% for header in headers %}<th scope="col">{{ header }}</th>{% endfor %}