Add support for implicit string type on string formating op

Add support for assigning a string using string formatting without first
declaring (assigning) as a string.

So instead of having to write
    FOOBAR=String()
    FOOBAR="%s and %s"%(FOO, BAR)
or
    FOOBAR=String("%s and %s"%(FOO, BAR))

you can just do
    FOOBAR="%s and %s"%(FOO, BAR)

and the parser will implicitly wrap it in a String() function call.

Signed-off-by: Esben Haabendal <esben@haabendal.dk>