Limit on max rows returned, controlled by --max_returned_rows option

If someone executes 'select * from table' against a table with a million rows
in it, we could run into problems: just serializing that much data as JSON is
likely to lock up the server.

Solution: we now have a hard limit on the maximum number of rows that can be
returned by a query. If that limit is exceeded, the server will return a
`"truncated": true` field in the JSON.

This limit can be optionally controlled by the new `--max_returned_rows`
option. Setting that option to 0 disables the limit entirely.

Closes #69

datasette/templates/database.html

@@ -30,6 +30,10 @@
     <p><input type="submit" value="Run SQL"></p>
 </form>
 
+{% if truncated %}
+    <div style="padding: 1em; margin: 1em 0; border: 3px solid red;">These results were truncated. You will need to apply OFFSET/LIMIT to see the whole result set.</div>
+{% endif %}
+
 {% if rows %}
 <table>
     <thead>

datasette/templates/error.html

@@ -8,7 +8,7 @@
 <h1 style="padding-left: 10px; border-left: 10px solid #{{ database_hash and database_hash[:6] }}">{{ database }}</h1>
 
 {% if error %}
-    <div style="padding: 1em; margin: 1em; border: 3px solid red;">{{ error }}</div>
+    <div style="padding: 1em; margin: 1em 0; border: 3px solid red;">{{ error }}</div>
 {% endif %}
 
 {% endblock %}