fixes related to embedded syntax for code blocks

There are 3 different fixes here:

1. Embedded syntax highlights did not work when a code block had an
   offset due to missing check for possible spaces in the "end" pattern
   for region "pandocDelimitedCodeBlock_" . s:langname.
2. Pandoc allows complex language preambles like
     ~~~~ {.cpp hl="vim" vars="PhHtmlEngine=tohtml,load_TagHl"}
   (an example taken from http://github.com/lyokha/vim-publish-helper).
   In this case if list g:pandoc_use_embeds_in_codeblocks_for_langs
   contains "vim" this will erroneously trigger highlights for vim
   though the block contains code from unlisted "cpp". This commit fixes
   this according to pandoc's rules for setting languages in code
   blocks.
3. The commit introduces value "2" for g:pandoc_syntax_fill_codeblocks:
   in this case embedded highlights stays Normal for normal items in
   code blocks with embedded syntax highlights whereas it is still
   Special for code blocks without them. This may increase readability
   in cases when there are many normal items in embedded code block.
   Probably it requires refactoring (use dedicated variable etc.) and it
   surely has to be mentioned in docs.