New extension: PubMed

Hi there,

Here's my first contribution to PopClip. 
And my first github contribution ever, actually :)

Hope everything looks fine in the files.

As stated by the official PubMed website: 
"PubMed comprises more than 23 million citations for biomedical literature from MEDLINE, life science journals, and online books. Citations may include links to full-text content from PubMed Central and publisher web sites."

This straightforward PopClip extension just uses a simple URL to search the selected text on Pubmed and open the search results in the default web browser. Several users have requested that PubMed has its own PopClip extension, so I hope people — especially scientists over the world— will find it useful!