add more strings functions support

[prostickman]

suggest add more func into the template.FuncMap where from "package strings"
likes "contains" "hasPrefix" and so on

[sriv]

should be available in nightly >= 13/4/2018

[nehashri]

The newly added string functions are :

"stringContains":      strings.Contains,
"stringHasPrefix":     strings.HasPrefix,
"stringHasSuffix":     strings.HasSuffix,
"stringJoin":          strings.Join,
"stringSplit":         strings.Split,
"stringCompare":       strings.Compare,
"stringReplace":       strings.Replace,
"stringTrim":          strings.Trim,
"stringToLower":       strings.ToLower,
"stringToUpper":       strings.ToUpper,
"stringToTitle":       strings.ToTitle,

[sriv]

also added sum that takes a slice of int and returns the sum.

[sswaroopgupta]

This feature has to be documented.

The docs for customization of html-report also needs to be added