title: Is it possible to append (rather than overwrite) cucumberOpt.tagExpressions in the command line?
body: <p>I have selenium tests which run using cucumber and wdio.
  My wdio.conf.js file looks like this:</p>
  
  <pre><code>cucumberOpts: {
      tagExpression: 'not @ignore',
  },
  </code></pre>
  
  <p>I have tests which I have tagged</p>
  
  <pre><code>@cats
  Scenario: As a user I want to google cats
  Given I have google open
  When I type in 'cats'
  And then I see pictures of cats
  
  @dogs
  Scenario: As a user I want to google dogs
  Given I have google open
  When I type in 'dogs'
  And then I see pictures of dogs
  
  @dogs @ignore
  Scenario: As a user I want to google fierce dogs
  Given I have google open
  When I type in 'fierce dogs'
  And then I see pictures of fierce dogs
  </code></pre>
  
  <p>If I want to run all dogs tests which haven't been ignored I can run:</p>
  
  <pre><code> ./node_modules/.bin/wdio wdio.conf.js --cucumberOpts.tagExpression='@dogs and not @ignore'
  </code></pre>
  
  <p>What I would like to be able to do is just run</p>
  
  <pre><code>./node_modules/.bin/wdio wdio.conf.js --cucumberOpts.tagExpression='@dogs'
  </code></pre>
  
  <p>(or similar) from the command line and have it pick up the 'not @ignore' from the wdio.conf.js file.</p>
  
  <p>Is it possible to do this?</p>