title: Remove columns from data frame if any row contains a specific string
body: <p>I would like to remove columns which contain the string <code>--</code> in any row.</p>
  
  <pre><code>Number  138 139 140 141 143 144 147 148 149 150 151 152 14  15  N…  
  nm4804  A   B   --  A   B   A   A   --  A   A   A   A   A   --  A  
  nm7574  B   A   A   A   A   A   A   A   A   A   A   A   A   --  A
  nm8723  B   --  B   B   B   --  A   --  B   B   B   B   --  --  A
  N…      B   A   A   A   A   B   A   --  A   A   B   --  --  --  A
  </code></pre>
  
  <p>I would like to count the -- frequency, if there is any column have more than 50% of -- in the columns, that column will be removed. </p>
  
  <p>Desired result:</p>
  
  <pre><code>Number  138 140 141 143 147 149 150 151 N…  
  nm4804  A   A   --    B A   A   A   A   A  
  nm7574  B   A   A    A  A   A   A   A   A
  nm8723  B   B   A    B  --    B  B  B   A
  N…          B   A   A    A  A A A   B   A
  </code></pre>
  
  <p><strong>Data (thanks bgoldst)</strong></p>
  
  <pre><code>df &lt;- data.frame(Number=c('nm4804','nm7574','nm8723','N…'),`138`=c('A','B','B','B'),`139`=c(
  'B','A','--','A'),`140`=c('--','A','B','A'),`141`=c('A','A','B','A'),`143`=c('B','A','B','A'
  ),`144`=c('A','A','--','B'),`147`=c('A','A','A','A'),`148`=c('--','A','--','--'),`149`=c('A',
  'A','B','A'),`150`=c('A','A','B','A'),`151`=c('A','A','B','B'),`152`=c('A','A','B','--'),
  `14`=c('A','A','--','--'),`15`=c('--','--','--','--'),`N…`=c('A','A','A','A'),check.names=F,
  stringsAsFactors=F);
  </code></pre>