title: Filter multiple columns by value using Checkbox group
body: <p>How can I use a checkbox group to filter several columns by value? 
  See example below: I want to return only the rows for which the columns indicated by the checkbox group have the value "yes".</p>
  
  <pre><code>---
  title: "test"
  runtime: shiny
  output: 
  flexdashboard::flex_dashboard:
  orientation: columns
  vertical_layout: fill
  ---
  
  ```{r setup, include=FALSE}
  library(flexdashboard)
  library(shiny)
  ```
  
  ```{r}
  checkboxGroupInput("checkGroup", label = h3("Checkbox group"), 
  choices = list("col1", "col2"))
  ```
  
  ```{r}
  df &lt;- data.frame(c(1,2,3),c("no","yes","yes"),c("no","no","yes"))
  colnames(df)&lt;-c("id","col1","col2")
  ```
  ```{r}
  renderDataTable({
      df        
  })
  ```
  </code></pre>
  
  <p>ie when selecting "col1', the output should contain only row row#2 and 3,
  when selecting "col2', the output should be: row#3,
  when selecting both "col1" and "col2", the output should be: row#2 and 3.</p>
  
  <p>I could write an if statement for each variable but I'd rather not (I have 10 or so). Surely there must be a better way?</p>