title: How to filter the retrieved items using a pnp.sp.web REST call to a SharePoint list
body: <p>I want to filter an SP list which my form is retrieving items from. The list has two columns Title, Start Date. I want to retrieve the start date items but dates that are either today or 21 days into the future.</p>
  
  <p>I have this already:</p>
  
  <pre><code> public _getMeetings() {
      pnp.sp.web.lists.getByTitle("Meetings").items.filter("StartDate What here?! eq 'WHAT HERE?!'").get().then((items: any[]) =&gt; {
        let returnedPMeetings: IDropdownOption[] = items.map((item) =&gt; { return { key: item.StartDate, text: item.StartDate }; });
        this.setState({ MeetingsList: returnedPMeetings });
      });
    }
  
  
  </code></pre>
  
  <p>As you can see I've got something in the filter arguments but I don't know how to achieve the specific filter I need. There are no decent resources on the syntax required for filtering that I know of.</p>