(.cmd.list): classify NULL as 'num' rather than 'kwa', aligning it with NA etc.
highlight.R (.operators): add `::` and `:::` to the operator list
Give `=` following formal arguments the same class with assignment operators
Adjust a test according to last commit.
Revert the last two commits about `=`
Merge pull request #3 from lq/master

Adjustments to the markup of a few tokens