Fix a typo in the Readme

It showed incorrect tag and the syntax was not in a code block
Import default markdown extensions from settings instead of readers

Closes getpelican/pelican-plugins#69

Due to change in pelican code, `import EXTENSIONS` was no longer
working.
Merge pull request #70 from talha131/fix-liquid-tag-issue-n-doc

Fix liquid tag that was broken due to change in pelican
Remove legacy STATIC_OUT_DIR from code

Closes getpelican/pelican-plugins#71
Remove incorrect folder from description in docs

Plugin reads files from `content/code/`, instead of
`content/downloads/code/`.
Merge pull request #72 from talha131/fix-liquid-tags-include_code

Remove legacy code in liquid tags include_code plugin