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'
Highlight spelling errors in headers.
Check spelling in setex style headers as well
Merge pull request #60 from mdlerch/spell_in_headers

Highlight spelling errors in headers.
Support delimited codeblocks in blockquotes

Fixes issue #58.

TODO: support embedded highlighting.
Support embeds in delimited codeblocks inside blockquotes

Completes fix for issue #58.
Allow unformatted text in blockquotes
remove blockquote codeblock in DisableEmbedsforCodeblocksWithLang
Removed reference to non-existing syntax group
only match > in codeblocks when the char is at the beggining of the line
prevent tables to override codeblocks

fixes issue #61
fix typos in commit f1d92df
Don't start pandocLaTeXInlineMath after an escaped $.
Update README

Should be clearer that vim-pandoc-syntax is supposed to be used
alongside vim-pantondoc.
Some fixes to emphasis inside strong, dashes, and list items

Also, updated tests