Update configuration.md

repeat warning about `destination` that is found in <http://jekyllrb.com/docs/usage/>
(see also pull request #3159)
Add jekyll-500px-embed to tag plugins
Fix a typo

Replaced "capaibilities" with correct spelling: "capabilities".
Fix erroneous 'next_section' value

According to /site/_data/docs.yml, the document next to
'deployment-methods' is 'continuous-integration', not 'troubleshooting'
(the 'prev_section' value in 'troubleshooting' is correctly set to
'deployment-methods' though).
Update configuration.md

`keep_files` is a very useful option when using tools such as `grunt-build-control`, `grunt`, `gulp` or `yeoman`.
Merge pull request #3165 from fabschurt/fix-doc-typos
Update history to reflect merge of #3165 [ci skip]
Merge pull request #3162 from dnozay/patch-4
Update history to reflect merge of #3162 [ci skip]
Merge pull request #3161 from dnozay/patch-3
Update history to reflect merge of #3161 [ci skip]
Merge pull request #3163 from lkorth/patch-1
Update history to reflect merge of #3163 [ci skip]
Add appveyor configuration YAML.
Tweak appveyor.yml.
Tests should run now.
README.markdown: Add AppVeyor badge.