Support for replace with regexp

This PR addresses issues schibsted#346 and schibsted#347.
Added documentation.

functions.md

@@ -523,9 +523,12 @@ If the `regexp` does not match the input,`out` corresponds to `value`.
 Examples:
 
 ```
-replace-regexp("2019-12-31", "([0-9][0-9][0-9][0-9])-([0-9][0-9])-([0-9][0-9])", "$2/$3/$1")                      => "12/31/2019"
-replace-regexp("2019-12-31", "(?<year>[0-9][0-9][0-9][0-9])-(?<month>[0-9][0-9])-(?<day>[0-9][0-9])", "$3.$2.$1") => "31.12.2019"
-replace-regexp("2019-12-31", "([a-z]+)", "$1")                                                                    => "2019-12-31"
+replace-regexp("2019-12-31", "([0-9][0-9][0-9][0-9])-([0-9][0-9])-([0-9][0-9])", "$2/$3/$1")
+   => "12/31/2019"
+replace-regexp("2019-12-31", "(?<year>[0-9][0-9][0-9][0-9])-(?<month>[0-9][0-9])-(?<day>[0-9][0-9])",
+   => "${day}.${month}.${year}") => "31.12.2019"
+replace-regexp("2019-12-31", "([a-z]+)", "$1")
+   => "2019-12-31"
 ```
 
 ### _trim(string) -> string_