Namespace configuration variables, use "pandoc" prefix

globals use the scheme

   g:pandoc#MODULE#VAR

buffer locals use

   b:pandoc_MODULE_VAR

General settings are namespaced into "modules" and "filetypes".
Initialize configuration default values when modules are initialialized

also, some style fixes.
Improved documentation
minor style fixes
doc: fix some mispellings and typos
Merge pull request #37 from vim-pandoc/next

IMPORTANT:CONFIG CHANGES: Preparation for vim-pandoc replacement

This changes all configuration variables names, so all users are advised to change their configuration to reflect this. The documentation (`:help pantondoc`) has been thoroughly revised and improved, so please refer to it when doing so.