contribution: new extension "PubMed"

I’ve just created the PubMed.popclipextz file (and related source files
in the /source folder). Hope you’ll like it and find it useful enough :)
source files added for new "PubMed" extension
updated Title String in PubMed extension

to correctly capitalize the PubMed name (previously: Pubmed)
Merge pull request #464 from pwizla/master

New extension: PubMed