title: Using Single date filter for date range in superset custom filter
body: <p>In Superset, I would like to use only single date selection but use that for range of date e.g. 7 days back.
  for example:
  Date selection : 2020-04-18  to 2020-04-18</p>
  <p>Expected date: 2020-04-11 to 2020-04-18
  Reason is that most dashboard required single date but some required time series which shows last 7 days result.</p>
  <p>Here is the code I prepare in custom query.</p>
  <pre><code>    SELECT  distinct(dt) FROM mytable
  WHERE 
      dt &gt;= date_add('day', -7, from_iso8601_timestamp('{{ to_dttm.isoformat() }}'))
      AND
      dt &lt;= date_add('day', 0, from_iso8601_timestamp('{{ to_dttm.isoformat() }}'))
  </code></pre>
  <p>this is result from json. It did put extra query WHERE, it runs pass but only 1 date shown.</p>
  <pre><code>SELECT distinct(dt)  FROM mytable\n
  WHERE dt &gt;= date_add('day', -7, from_iso8601_timestamp('2020-04-18T00:00:00'))\n     
  AND dt &lt;= date_add('day', 0, from_iso8601_timestamp('2020-04-18T00:00:00'))) AS \&quot;expr_qry\&quot;\n
  WHERE \&quot;dt\&quot; &gt;= from_iso8601_timestamp('2020-04-18T00:00:00')\n  
  AND \&quot;dt\&quot; &lt;= from_iso8601_timestamp('2020-04-18T00:00:00')\nLIMIT 1000&quot;
  </code></pre>