Deduplicate syntax keyword arguments

This is a bug fix / improvement to the new syntax keyword usage
introduced in b6f8757d004d5f4ef7280fd111a21821e6bee79a.

Also relevant is issue #68 on GitHub, see
https://github.com/xolox/vim-easytags/issues/68
Filter out forbidden syntax keyword arguments

This is a bug fix / improvement to the new syntax keyword usage
introduced in b6f8757d004d5f4ef7280fd111a21821e6bee79a.

Also relevant is issue #68 on GitHub, see
https://github.com/xolox/vim-easytags/issues/68
Bug fix: Restore default syntax pattern prefix/suffix (word boundaries)

This is a bug fix / improvement to the new syntax keyword usage
introduced in b6f8757d004d5f4ef7280fd111a21821e6bee79a.

Also relevant is issue #68 on GitHub, see
https://github.com/xolox/vim-easytags/issues/68

@tarmack: Thanks for the feedback earlier today, it's now fixed! :-)