Preview post added to index

[jmglov]

Posts with

Preview: true

in their metadata are not meant to be published, and therefore should be omitted from the following places:

• index.html
• archive.html
• tags/*.html
• RSS feeds (atom.xml and planetclojure.xml)

Preview posts are added to index.html.

Repro steps

bb quickblog new --file test-preview.md --title 'preview test'
vim posts/test-preview.md  # add Preview: true
bb quickblog render                                    
grep -rn test-preview public/ | cut -d ':' -f 1 | sort | uniq
# =>
# public/index.html
# public/test-preview.html

[jmglov]

Picking this one up