add support for code embedded in delimited codeblocks.

should implement partially a fix to issue #14.
fix pandocDelimiteCodeBlockEnd not being concealed in some cases.

also, removed cmake from supported languages.
use Special for codeblocks highlighting.
make codeblock fills optional
add codeblock embedded language highlighting support

Clears syn sync manipulations made by embedded languages at the end, to
avoid syn sync bugs in the overall document (as observed with c.vim
comments).
Merge branch 'codeblock-embeds' of github.com:vim-pandoc/vim-pandoc-syntax
renamed s:pandoc_syntax_cchars to s:cchars
added configurability to embedded highlighting for codeblocks.

created functions to enable and disable highlighting for specific langs.