title: mapview error in knitr R markdown document
body: <p>I've been playing around with the <strong>mapview</strong> package which has recently been loaded onto <a href="https://cran.r-project.org/package=mapview" rel="nofollow">CRAN</a>. I've been playing with the demo code and trying to embed it into a <strong>knitr</strong> <strong>markdown</strong> document. Unfortunately, I get an error message when I do this and I'm having trouble interpreting what is going wrong.</p>
  
  <p>Here is the error message, followed by a reproducible example. Note that I don't have any problem when running the code as straight-up R code. It's only when it is run through <strong>knitr</strong> that the problem presents itself. The error does not occur if the <code>mapview</code> elements are commented out.</p>
  
  <p>I've updated all of my packages and the problem persists. Here are some system details:</p>
  
  <ul>
  <li>Windows 7</li>
  <li>R version 3.2.2</li>
  <li>R Studio version 0.99.486</li>
  <li>mapview version 1.0.0</li>
  <li>knitr version 1.11</li>
  <li>pandoc version 1.13.1</li>
  </ul>
  
  <blockquote>
    <p>pandoc.exe: Could not fetch
    C:\Users\my.name\Documents\R\win-library\3.2\mapview\htmlwidgets\lib\leaflet#default#VML
    C:\Users\my.name\Documents\R\win-library\3.2\mapview\htmlwidgets\lib\leaflet:
    openBinaryFile: does not exist (No such file or directory) Error:
    pandoc document conversion failed with error 67 In addition: Warning
    message: running command '"C:/Program Files/RStudio/bin/pandoc/pandoc"
    +RTS -K512m -RTS Mapview.utf8.md --to html --from markdown+autolink_bare_uris+ascii_identifiers+tex_math_single_backslash-implicit_figures
    --output Mapview.html --smart --email-obfuscation none --self-contained --standalone --section-divs --template "C:\Users\my.name\Documents\R\win-library\3.2\rmarkdown\rmd\h\default.html"
    --variable "theme:bootstrap" --include-in-header "C:\Users\my.name\AppData\Local\Temp\Rtmpw9Mi9D\rmarkdown-str1ee41c515f3f.html"
    --mathjax --variable "mathjax-url:<a href="https://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML" rel="nofollow">https://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML</a>"
    --no-highlight --variable "highlightjs=C:\Users\my.name\Documents\R\win-library\3.2\rmarkdown\rmd\h\highlight"'
    had status 67  Execution halted</p>
  </blockquote>
  
  <pre><code>---
  title: "Test Mapview"
  output: html_document
  ---
  
  
  ```{r}
  
  library(ggplot2)
  library(rgdal)
  library(mapview)
  
  data(diamonds)
  
  ### blow diamonds up a bit
  big &lt;- diamonds[rep(seq_len(nrow(diamonds)), 1), ]
  big$cut &lt;- as.character(big$cut)
  big$color &lt;- as.character(big$color)
  big$clarity &lt;- as.character(big$clarity)
  
  ### provide some random positions
  big$x &lt;- rnorm(nrow(big), 0, 10)
  big$y &lt;- rnorm(nrow(big), 0, 10)
  coordinates(big) &lt;- ~x+y
  proj4string(big) &lt;- CRS("+init=epsg:4326")
  
  ### view it
  mapview(big)
  
  ```
  </code></pre>