Don't rewrite robots.txt

Leave `robots.txt` alone, esp. so we can point robots to the proper sitemap, if we have multi-language setup with prepended URLs, e.g. `/de/sitemap.xml`
Merge pull request #79 from jng5/patch-1

Don't rewrite robots.txt
Allow to format the site's modified timestamp (like for pages).
Merge pull request #80 from danielweidner/consistency

Allow to format the site's modified timestamp
added m4a in the extensions list
Merge pull request #91 from mcbrwr/master

added m4a in the extensions list
Better file existence check.

Remove query and hash from the URI before using it as file name.
Merge pull request #100 from Zegnat/relativity-patch

Improving url() compatibility with local files.
Merge branch 'master' of github.com:bastianallgeier/kirbycms