Added pdb filetype

This is really nothing, but vim used that extension not to confuse prolog and perl, and it was boring to manually say which grammar my file were each time I edited some prolog file.
Merge pull request #2 from iSheeft/master

New operators
Merge pull request #3 from Artymort/master

Added pdb filetype
Prepare 0.4.0 release
Update README.md
Update README.md
Added support for pl1 as requested