db.retrieve match args for db.FILE, db.FLAG, etc

Search functionality added to db.retrieve:

return flags in file '/home/user/src/hello.py' with db.retrieve(db.FILE,'/home/user/src/hello.py')

return flags in all files corresponding to flag type TODO with db.retrieve(db.FLAG,'TODO')

return flags in file '/home/user/src/goodbye.c' corresponding to flag type NEEDS-REVISION with db.retrieve(db.FILE_AND_FLAG,['/home/user/src/goodbye.c','NEEDS-REVISION']) <---note the list this time.