Fix option of turning foldtext on/off.

1. Exit folding.vim when g:LatexBox_Folding is 0 or undefined.
2. Initialize all of the options the their default values at first when
   g:LatexBox_Folding is on.
Merge pull request #188 from cwahbong/foldtext_fix

Fix option of turning foldtext on/off (fixes #187)