IMPORTANT: Namespace configuration variables

IMPORTANT: User might need to change his configuration. See :h vim-
pandoc-syntax-configuration

All configuration variables are now under the g:pandoc#syntax namespace.

We are now following the new directives in vim-pantondoc (see vim-pandoc
/vim-pantondoc#34), so g:pandoc_use_conceal turns into
g:pandoc#syntax#conceal#use, etc.