title: Using flextable in r markdown loop not producing tables
body: <p>I have many tables to create and am trying to create them in a loop. I'm using flextable with rmarkdown inside rstudio.  Using <code>print(theFlextable)</code> command in a loop produces a list of text rather than the table.  This happens for docx and html output types. If I don't use a loop flextable renders correctly. Here is a demo:</p>
  
  <pre><code>---
  title: "Demo"
  output: word_document
  ---
  
  ```{r setup, include=FALSE}
  library(flextable)
  ```
  ## This Works
  ```{r iris, echo=F, message=F, error=F, results='asis'}
  ft&lt;-flextable(iris[1:10,])
  ft
  ```
  ## This produces no output
  ```{r echo=F, message=F, error=F, results='asis'}
  doThese&lt;-c("setosa","virginica")
  for (i in doThese){
    tbl&lt;-subset(iris, Species==i)
    ft&lt;-flextable(tbl[1:10,])
    ft
  }
  ```
  ## This produces incorrect output
  ```{r echo=F, message=F, error=F, results='asis'}
  doThese&lt;-c("setosa","virginica")
  for (i in doThese){
    tbl&lt;-subset(iris, Species==i)
    ft&lt;-flextable(tbl[1:10,])
    print(ft)
    cat("\n\n")
  }
  ``` 
  </code></pre>
  
  <p>This is the output in word of the last block above:</p>
  
  <p>type: flextable object. col_keys: Sepal.Length, Sepal.Width, Petal.Length, Petal.Width, Species header has 1 row(s) body has 10 row(s) original dataset sample: Sepal.Length Sepal.Width Petal.Length Petal.Width Species 1 5.1 3.5 1.4 0.2 setosa 2 4.9 3.0 1.4 0.2 setosa 3 4.7 3.2 1.3 0.2 setosa 4 4.6 3.1 1.5 0.2 setosa 5 5.0 3.6 1.4 0.2 setosa</p>
  
  <p>type: flextable object. col_keys: Sepal.Length, Sepal.Width, Petal.Length, Petal.Width, Species header has 1 row(s) body has 10 row(s) original dataset sample: Sepal.Length Sepal.Width Petal.Length Petal.Width Species 101 6.3 3.3 6.0 2.5 virginica 102 5.8 2.7 5.1 1.9 virginica 103 7.1 3.0 5.9 2.1 virginica 104 6.3 2.9 5.6 1.8 virginica 105 6.5 3.0 5.8 2.2 virginica</p>