title: Pandas Merge 2 dataframe if one of the values are same
body: <p>I have 2 dataframes:</p>
  <p><code>df1</code>:</p>
  <pre><code>     id            color
  0    005           green
  1    005           blue
  </code></pre>
  <p><code>df2</code>:</p>
  <pre><code>    user_id         name
  0    005            eric
  </code></pre>
  <p>I'm trying to user <code>pd.dataframe.merge</code> to merge these 2 dataframes</p>
  <pre><code>df2 = df2.merge(df1, left_on='user_id', right_on='id')[['id', 'name', 'color']]
  </code></pre>
  <p>I got the following result:</p>
  <p><code>df2</code>:</p>
  <pre><code>      id       name     color
  0    005       eric     green
  1    005       eric     blue
  </code></pre>
  <hr/>
  <p>Are there ways for me to combine these 2 columns into 1? for example:</p>
  <p><code>df2</code>:</p>
  <pre><code>      id       name     color
  0    005       eric     green,blue
  </code></pre>