do not force the permalink to be a dir if it ends on .html

Fixes #798.
test per-post permalinks

Add tests for setting a permalink on a post (in YAML frontmatter),
both the "old way" (generating an index.html) and with an .html ex-
tension.
use binary mode when writing file.
Add jekyll-ditaa to plugins list
Merge pull request #1370 from matze/add-ditaa-plugin

Add jekyll-ditaa to plugins list
Update history to reflect merge of #1370
Add postfiles plugin

Add my postfiles plugin to the list of plugins.
Escape tag literal
Merge pull request #1373 from indirect/patch-1

Add postfiles plugin
Update history to reflect merge of #1373.
add test to demonstrate issue with markdown extension parsing. adding mkd as an extension should not cause mkdn files to also be markdownified
force markdown match to match the full extension
Only generate excerpts if the excerpt_separator isn't blank
We don't need excerpts in jekyllrb.com
bugs in the site
Just clarifying the Post#should_generate_excerpt?
Docs about disabling excerpts.
It's WordPress - capital P, dang it.

WordPress, like GitHub, is a camel-cased name that deserves to be
written properly. Props to @jeremyvisser for pointing this out.

Closes #1384
Merge pull request #1382 from torsneyt/fix_md_ext

Markdown extension matching matches only exact matches
Update history to reflect merge of #1382