Use ruby-mode for Podfile

A Podfile is used for CocoaPods to do a Bundler like dependency
management in XCode projects. Similar to a Rakefile or a Gemfile the
Podfile contains a dependency description in a Ruby based DSL.
Factor the search engine installation to ease the future install of new ones.
Add DuckDuckGo search engine in the default list.
Add binding `C-c U` for DuckDuckGo search
web-mode is now compatible with minor modes
[Fix #484] Set OS X specific latex settings conditionally
Bootstrap module use
Delegate loading to prelude-packs