add a new syntax keyword support: mermaid

Add unicode test case into test.md

fix a issue when the table is not support

v3.1.2 release

fix bugs when the passage has unicode characters

plugin/markdown-preview.vim

README.md

Merge pull request #10 from 513266358/master

modify unicode(utf-8) support python3

fix bug when the vim has no python2

v3.0.1 release

1. upgrade CommonMark to the newest.
2. remove the warning when preview unicode files

upgrade to v3.0.0

1. add CommonMark support
2. bug fixes