clear local variables for the first execution of mrb_context_run(); close #2405
Syntax highlight code blocks with github flavored markdown.
Merge pull request #2409 from take-cheeze/syntax_highlight

Syntax highlight code blocks with github flavored markdown.
Fix document of doc/mrbconf/README.md
Fix a document in doc/mrbgems/README.md
Fix document of  doc/compile/README.md
Merge pull request #2410 from suzukaze/Fix-docs

Fix document of doc/mrbconf/README.md
Merge pull request #2411 from suzukaze/Fix-docs2

Fix document in doc/mrbgems/README.md
Merge pull request #2412 from suzukaze/Fix-doc3

Fix document of  doc/compile/README.md