Add support for multi-element vector swizzles. Issue #17.

This commit adds support for swizzles like "foo.zy" (if "foo" is,
for example, a float<3> type) as rvalues.  (Still need support for
swizzles as lvalues.)

lex.ll

@@ -72,6 +72,7 @@ FLOAT_NUMBER (([0-9]+|(([0-9]+\.[0-9]*[fF]?)|(\.[0-9]+)))([eE][-+]?[0-9]+)?[fF]?
 HEX_FLOAT_NUMBER (0x[01](\.[0-9a-fA-F]*)?p[-+]?[0-9]+[fF]?)
 
 IDENT [a-zA-Z_][a-zA-Z_0-9]*
+ZO_SWIZZLE ([01]+[w-z]+)+|([01]+[rgba]+)+|([01]+[uv]+)+
 
 %%
 "/*"            { lCComment(yylloc); }