correct cref reference filter

the duplicate cref entries always appear with `@cref` at the end of the entry. gVim does not interpret the previous regex as expected (gVim 7.4.346 under Windows 7 64 bit).
Merge pull request #199 from EPNGH/patch-4

Correct cref reference filter
Fix undefined fold_toc option (fixes #202)

Also added comments to explain two new options.