Search for Categories and Tags, simplified search-result container

[lorenzschmid]

Allowed myself to add some new functionalities and bump (at least) some of the code to the new Jekyll's v3 structure (documents handle attributes such as categories and tags differently). Following changes got added in this PR (possible issue fixes are mentioned):

• Categories are included in index: You can now search for it and use them in the mustache template
• Tags are included in index: You can now search for it and use them in the mustache template (fixes #90)
• Added additional attribute is_post to index to use in mustache template (indicates if entry is a post/document, fixes #78)
• Simplified search-result container: Instead of having two container (one for the results and one for the headline "Search results" further containing the first container) there is now only one and a headline can be prepended to the results via JavaScript if desired (new configure value titleMsg)
• Changed the two configuration message values (titleMsg and emptyMsg) to also contain their container in form of HTML tags instead of only being text/string
• Finally removed the search.js.map generation (can still be generated in the Rakefile with a specific function) to ease integration for production use (fixes #51)

BTW: Thanks for the easy production and building setup, very convenient!

[slashdotdash]

@lorenzschmid Sweet PR, thanks

[lorenzschmid]

Thanks, you're welcome!