title: passing arguments to jq filter
body: <p>Here is my config.json:</p>
  
  <pre><code>{
      "env": "dev",
      "dev": {
          "projects" : {
              "prj1": {
                  "dependencies": {},
                  "description": ""
              }
          }
      }
  }
  </code></pre>
  
  <p>Here are my bash commands:</p>
  
  <pre><code>PRJNAME='prj1'
  
  echo $PRJNAME
  
  jq --arg v "$PRJNAME" '.dev.projects."$v"' config.json 
  jq '.dev.projects.prj1' config.json 
  </code></pre>
  
  <p>The output:</p>
  
  <pre><code>prj1
  null
  {
    "dependencies": {},
    "description": ""
  }
  </code></pre>
  
  <p>So $PRJNAME is prj1, but the first invocation only outputs <code>null</code>.</p>
  
  <p>Can someone help me?</p>