Basic CSV export, refs #266

Tables and custom SQL query results can now be exported as CSV.

The easiest way to do this is to use the .csv extension, e.g.

	/test_tables/facet_cities.csv

By default this is served as Content-Type: text/plain so you can see it in
your browser. If you want to download the file (using text/csv and with an
appropriate Content-Disposition: attachment header) you can do so like this:

	/test_tables/facet_cities.csv?_dl=1

We link to the CSV and downloadable CSV URLs from the table and query pages.

The links use ?_size=max and so by default will return 1,000 rows.

Also fixes #303 - table names ending in .json or .csv are now detected and
URLs are generated that look like this instead:

	/test_tables/table%2Fwith%2Fslashes.csv?_format=csv

The ?_format= option is available for everything else too, but we link to the
.csv / .json versions in most cases because they are aesthetically pleasing.

datasette/templates/query.html

@@ -40,7 +40,7 @@
 </form>
 
 {% if rows %}
-<p>This data as <a href="{{ url_json }}">.json</a></p>
+<p class="export-links">This data as <a href="{{ url_json }}">JSON</a>, <a href="{{ url_csv }}">CSV</a> (<a href="{{ url_csv_dl }}">download CSV</a>)</p>
 <table class="rows-and-columns">
     <thead>
         <tr>