title: how to use select for multiple fields using dplyr
body: <p>I have a character vector of field names that I want to select using dplyr.  I'm using the underscore version of select_().</p>
  
  <pre><code>select(mtcars, mpg)                   # works OK
  select(mtcars, mpg, disp, am)         # works OK for multiple fields
  </code></pre>
  
  <p>now let's use the underscore version</p>
  
  <pre><code>fie &lt;- c("mpg")             
  select_(mtcars, fie)                  # works OK for one
  fie &lt;- c("mpg", "disp", "am")
  select_(mtcars, fie)                  # problem:  only returns one column
  select_(mtcars, ~fie)                 # problem:  doesn't work
  </code></pre>
  
  <p>I'm confused as to how to get this to work. Any suggestions?
  Thanks </p>