Add Concat/Join/Store helpers

[gfanton]

• concat: Concatenates the elements passed as argument to create a single string.
• join: Same as concat but take a separator as fist argument.
• store: Store anything, useful to pass some vars across multiple files/scopes

[moul]

FYI, join is already shipped with sprig: https://github.com/Masterminds/sprig/blob/master/functions.go#L196