'index' func overwrites default 'index'

[Etomyutikos]

The strings.Index func added to the func map in environ/funcs.go overwrites the text/template index func, making some operations impossible.

[natefinch]

Hugo uses namespaces for functions. I think this is the right fix. Implementation looks like this:

https://play.golang.org/p/TIV-2BEO1o