Add reCaptcha support

[ccamel]

This PR adds reCaptcha support as an additional security layer against unwanted / spam form submission.

This is a fairly straighforward implementation which verifies the challenge code in the g-recaptcha-response POST parameter, if available.

Feel free to make any comment on this one. 😌

[rchakode]

yo,
Well done, this is a great improvment on security to prevent spams/scam mails.
Let me test this on my side before merging it.

[rchakode]

Hello @ccamel ,
I gave a try to the PR, it works very well. 👍
I would suggest the following changes:

• remove the template file samples/hugo-partial-sample-form-recaptcha.html, and,
• include the content to the default template as HTML comments, as below
  I think that make it easy to integrate

<div id="reply-message"></div>
<div>
   <fieldset>
      <legend>Please fill in the form to submit your request</legend>
      <form action="https://contact-request-endpoint/" method="post">
         <!-- uncomment this div block when enabling reCaptcha
         <script src="https://www.google.com/recaptcha/api.js"></script>
         -->         
         <div class="form-item">
            <label for="name">Name</label>
            <input type="text" name="name" id="name"  placeholder="Mr. Smith" />
         </div>
         <div class="form-item">
            <label for="email">Email <span class="req"></span></label>
            <input type="text" name="email" id="email"  class="required email" placeholder="[email protected]" />
         </div>
         <div class="form-item">
            <label for="organization">Organization</label>
            <input type="text" name="organization" id="organization"  placeholder="Company, Inc." />
         </div>
         {{ if in .Params.tags "contact" }}  
         <div class="form-item">
            <label for="subject">Subject</label>
            <input type="text" name="subject" id="subject"  value="" placeholder="Need help or expertise?" />
            <input type="hidden" name="target" id="target"  value="contact" />
         </div>
         <div class="form-item">
            <label for="message">Message</label>
            <textarea  rows="6" name="message" id="message"  placeholder="Please add details concerning your request."></textarea>
         </div>
         {{ else }}    
         <div class="form-item">
            <input type="hidden" name="subject" id="subject"  value="Your Access to Product Demo!" />
            <input type="hidden" name="target" id="target"  value="demo" />
         </div>
         {{ end }}
         <!-- uncomment the below div when enabling reCaptcha
         <div class="g-recaptcha" data-sitekey="{{.Site.Params.reCaptchaPrivateKey}}"></div>
         -->
         <input  class="button"  type="submit" value="Submit">
      </form>
   </fieldset>
</div>

[ccamel]

@rchakode I agree! I've made the change in that way. 😌
Thanks.