Add index.xml to WEBrick DirectoryIndex

Resolves #2040
Merge pull request #2041 from anthonyjsmith/webrick_index_xml
Update history to reflect merge of #2041 [ci skip]
Make LayoutReader#layout_directory public.

Since it's being used in a test, let's go ahead and expose it instead of
dealing with using Object#send to send the message along.
Merge pull request #2058 from jekyll/layouts-relative-to-config
Update history to reflect merge of #2058 [ci skip]
Move Kramdown version bump to Minor Enhancements section.

Closes #2069.
[configuration.md] Correct typo

Typo: 'use'
Proposed Correction: 'uses the'
---
In English, 'use' is the verb form for plural subjects, where as 'uses' is the verb form for a singular subject. For this case, the subject, 'default value', is singular.
Merge pull request #2073 from mizuhands/patch-1
Update history to reflect merge of #2073 [ci skip]
Update configuration docs to indicate that utf-8 is now the default

Docs for #2031, /cc #2029.
Expose site.static_files to Liquid
Test fetching of static files
Test site.static_files
Add StaticFile#to_liquid and StaticFile#relative_path
Add docs for site.static_files
Fix error in tests.
Add StaticFile#to_liquid['extname'] :)
Sort the static files by relative path before sending to liquid
Update docs to include extname for static file object in liquid. [ci skip]