title: command line comparison (e.g. diff) ignoring non-whitespace first character of each line while keeping displaying it in output
body: <p>pandoc prints the available extensions for each format with a leading <code>+</code> or <code>-</code> (depending on whether the extension is enabled or disabled). So for example</p>
  
  <pre><code>$ pandoc --list-extensions=markdown
  -abbreviations
  +all_symbols_escapable
  -angle_brackets_escapable
  +auto_identifiers
  -autolink_bare_uris
  +backtick_code_blocks
  ...
  </code></pre>
  
  <p>In order to diff two format's available extensions, I simply do (bash)</p>
  
  <pre><code>$ diff -y &lt;(pandoc --list-extensions=markdown | tr -d '+-') &lt;(pandoc --list-extensions=commonmark | tr -d '+-')
  abbreviations                                                 &lt;
  all_symbols_escapable                                           all_symbols_escapable
  angle_brackets_escapable                                      | ascii_identifiers
  auto_identifiers                                                auto_identifiers
  autolink_bare_uris                                              autolink_bare_uris
  backtick_code_blocks                                            backtick_code_blocks
  blank_before_blockquote                                       &lt;
  blank_before_header                                           &lt;
  </code></pre>
  
  <p>In the output I'd like to keep the leading <code>+</code> or <code>-</code>, that is for example</p>
  
  <pre><code>+abbreviations                                                 &lt;
  +all_symbols_escapable                                           -all_symbols_escapable
  -angle_brackets_escapable                                      | -ascii_identifiers
  +auto_identifiers                                                -auto_identifiers
  -autolink_bare_uris                                              -autolink_bare_uris
  +backtick_code_blocks                                            -backtick_code_blocks
  +blank_before_blockquote                                       &lt;
  +blank_before_header                                           &lt;
  </code></pre>
  
  <p>Please notice that this is different from running</p>
  
  <pre><code>$ diff -y &lt;(pandoc --list-extensions=markdown) &lt;(pandoc --list-extensions=commonmark)
  -abbreviations                                                | -all_symbols_escapable
  +all_symbols_escapable                                        | -ascii_identifiers
  -angle_brackets_escapable                                     | -auto_identifiers
  +auto_identifiers                                             &lt;
  -autolink_bare_uris                                             -autolink_bare_uris
  +backtick_code_blocks                                         | -backtick_code_blocks
  +blank_before_blockquote                                      &lt;
  +blank_before_header                                          &lt;
  </code></pre>
  
  <p>What's the simplest way of doing it?</p>