New sortable_columns option in metadata.json to control sort options

You can now explicitly set which columns in a table can be used for sorting
using the _sort and _sort_desc arguments using metadata.json:

    {
        "databases": {
            "database1": {
                "tables": {
                    "example_table": {
                        "sortable_columns": [
                            "height",
                            "weight"
                        ]
                    }
                }
            }
        }
    }

Refs #189

datasette/templates/table.html

@@ -8,7 +8,7 @@
 <style>
 @media only screen and (max-width: 576px) {
 {% for column in display_columns %}
-    td:nth-of-type({{ loop.index }}):before { content: "{{ column|escape_css_string }}"; }
+    td:nth-of-type({{ loop.index }}):before { content: "{{ column.name|escape_css_string }}"; }
 {% endfor %}
 }
 </style>