Add altmetric badges to paper (alshedivat#950)

[Altmetric](https://www.altmetric.com/) tracks social media and citation
activity for a paper. I added an option to add the altmeric id to a
paper's bib entry, which results in the altmeric badge for the paper
being displayed next to the other buttons. As an example, I added the
altmetric id for the Einstein, Podolsky & Rosen paper.

README.md

@@ -421,6 +421,7 @@ There are several custom bibtex keywords that you can use to affect how the entr
 - `poster`: Adds a "Poster" button redirecting to a specified file (if a full link is not specified, the file will be assumed to be placed in the /assets/pdf/ directory)
 - `slides`: Adds a "Slides" button redirecting to a specified file (if a full link is not specified, the file will be assumed to be placed in the /assets/pdf/ directory)
 - `website`: Adds a "Website" button redirecting to the specified link
+- `altmetric`: Adds an [Altmetric](https://www.altmetric.com/) badge (Note: only add the altmetric identifier here - the link is generated automatically)
 
 You can implement your own buttons by editing the bib.html file.
 

_layouts/bib.html

@@ -156,6 +156,9 @@
           {%- if entry.website %}
             <a href="{{ entry.website }}" class="btn btn-sm z-depth-0" role="button">Website</a>
           {%- endif %}
+          {%- if entry.altmetric %}
+            <script type="text/javascript" src="https://d1bxh8uas1mnw7.cloudfront.net/assets/embed.js"></script><div data-altmetric-id="{{ entry.altmetric }}" class="altmetric-embed" data-hide-no-mentions="true" data-hide-less-than="15" data-badge-popover="right" data-badge-type="2" style="display:inline"></div>
+          {%- endif %}
           </div>
 
           {% if entry.abstract -%}