Proper 'define' support

Guessing I'm going to get some push back on the iskeyword tweak, but man
is this useful.  Press [d on a variable to see its value.  Press [<C-D>
on a mixin to jump to its definition.  Placeholder classes and functions
work too.

ftplugin/sass.vim

@@ -9,15 +9,18 @@ if exists("b:did_ftplugin")
 endif
 let b:did_ftplugin = 1
 
-let b:undo_ftplugin = "setl com< cms< def< inc< inex< ofu< sua<"
+let b:undo_ftplugin = "setl com< cms< def< inc< inex< isk< ofu< sua<"
 
 setlocal comments=://
 setlocal commentstring=//\ %s
-setlocal define=^\\s*\\%(@mixin\\\|=\\)
 setlocal includeexpr=SassIncludeExpr(v:fname)
+setlocal iskeyword+=-
+setlocal iskeyword+=$
+setlocal iskeyword+=%
 setlocal omnifunc=csscomplete#CompleteCSS
 setlocal suffixesadd=.sass,.scss,.css
 
+let &l:define = '^\C\v\s*%(\@function|\@mixin|\=)|^\s*%(\$[[:alnum:]-]+:|[%.][:alnum:]-]+\s*%(\{|$))@='
 let &l:include = '^\s*@import\s\+\%(url(\)\=["'']\='
 
 function! SassIncludeExpr(file) abort