Prevent 'racketSymbol' matching inside Niji_paren_level[n]

This fixes a case where users of the 'vim-racket' static runtime files
experience unwanted highlighting of characters followed by a '+'.

Niji's 'Niji_paren[n]' and 'Niji_paren_level[n]' take precendence over
'Delimiter' and 'racketStruc', respectively.  This is causing
'racketSymbol' to match where it shouldn't.  'racketSymbol' is defined as
follows:

	syn match racketSymbol    ,\k+,  contained

In other words, any keyword character followed by a '+'.  It is
'contained' in the 'racketQuotedStuff' syntax cluster, made up of the
'racketQuotedStruc' and 'racketUnquote' syntax regions.

Unfortunately, Niji's use of 'ALLBUT' in its syntax region matching also
'contains' 'racketSymbol', causing it to match.

Niji 'uses the ALLBUT syntax to exclude all parentheses except the next
level', resolving 'a bug found in all previous implementations [of
rainbow parentheses] where there is a single extra level of un-styled
parentheses.'

For now, I have special-cased the exlusion of 'racketSymbol'; rewriting
the matching method should be investigated.

Fixes #22.