Fixes issue #50

All credit goes to https://github.com/lyokha
Limit all lookbehinds to 1 byte, add display flag to many groups

The goal here is to improve performance.
Merge branch 'master' of github.com:vim-pandoc/vim-pandoc-syntax
Fix issue #51.
Fix issue #53.

Faulty look behind limit disabled embedded codeblock functionality.
Only limit look behinds when their lenght is determinate.

Should fix faulty look behinds in issue #53.
Merge remote-tracking branch 'upstream/master'
Re-appplyng fix to issue #50.
Merge remote-tracking branch 'upstream/master'
Detect endquote only before spaces, punctuation and newlines.

Should fix issue #49.
Prevent pandocEmphasisInStrong to begin with an escaped _ or *

Fixes issue #55
Merge remote-tracking branch 'upstream/master'
Fix issue #56

allow stuff to occur after bars at the end of pipe tables.
Check spelling on parenthetical citations, but don't use spelling for citekeys.

fixes issue #57.
Do not conceal ‘#’ in heading text
Trailing # signs are part of the syntax
Merge pull request #59 from vbgl/hash-in-header

Fix issue #59, re: Hash sign in headers
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.