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.
be silent if specific syntax file is missing
langname must match as a word
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
inline code in emphasis and strong areas

I often use inline code (double or single backticks) inside emphasis
areas: pandoc supports this and renders this as expected. But vim syntax
highlight may get broken as soon as emphasis and strong areas contain
inlined Latex math, but do not contain inline codes. It is enough to
insert dollar sign inside the code to make the whole pandoc syntax
highlight after it broken. This commit fixes this by allowing inline
codes in these areas.

Some implementation details. I did not use WithConceal() for inlined
code inside emphasis/strong areas because when using sole concealends
the backticks disappear and inline code in emphasis gets
indistinguishable from simple emphasis area. Another approach: using
concealends with cchar only shows ending characters (looks like a bug in
vim). Not concealing backticks make them visible and distinguishable as
they are not marked with relative italic or bold markers.
Merge pull request #78 from lyokha/master

inline code in emphasis and strong areas