title: Markdown not finding params when knitting
body: <p>When I try to knit the following document</p>
  
  <pre><code>----
  title: "jerHW52"
  output: html_document
  params:
    missiontype: "Successful.Unmanned"
  ---
  
  ```{r warning=FALSE,echo=FALSE}
  knitr::opts_chunk$set(echo=FALSE)
  library(tidyr)
  library(ggplot2)
  Launch_Data&lt;-read.csv("NASA_Launch_Data.csv",header=TRUE)
  NASA_Launch_Data&lt;- Launch_Data %&gt;% filter(Launch_Data$Type==params$missiontype)
  ggplot(NASA_Launch_Data, aes(x=Year, y=Missions)) + geom_line() 
  ```
  </code></pre>
  
  <p>I get the following error</p>
  
  <blockquote>
    <p>Error in filter(., Launch_Data$Type == params$missiontype) : object
    'params' not found Calls:  ... _fseq -> freduce ->
    withVisible ->  -> filter Execution halted</p>
  </blockquote>
  
  <p>Why would it not be able to find params?</p>
  
  <p>Here is the start of the data in NASA_Launch_Data.csv</p>
  
  <pre><code>"","Year","Type","Missions"
  "1",1957,"Successful.Unmanned",3
  "2",1958,"Successful.Unmanned",28
  "3",1959,"Successful.Unmanned",23
  "4",1960,"Successful.Unmanned",40
  "5",1961,"Successful.Unmanned",48
  "6",1962,"Successful.Unmanned",76
  "7",1963,"Successful.Unmanned",67
  "8",1964,"Successful.Unmanned",99
  "9",1965,"Successful.Unmanned",119
  "10",1966,"Successful.Unmanned",129
  </code></pre>