fix full path leak to source directory when using include tag
added tests for full path leak to source directory when using include tag
Update post.md with additional info about excerpt

Adds a paragraph noting that use of the liquid `| strip_html` flag is possible and useful for meta tag purposes.
small tweak to clarify last sentence
make sure pages with published being false are not generated
Merge pull request #1962 from jglovier/patch-1
Update history to reflect merge of #1962 [ci skip]
Merge pull request #1951 from jens-na/include-tag-error-path
Update history to reflect merge of #1951 [ci skip]
Merge pull request #1931 from liufengyun/published
Update history to reflect merge of #1931