`GroupBy` is modified to allow it to receive a method name argument for
example `Type` as its first argument. It is only allowed to call with
a method which takes no arguments and returns a result or a pair of
a result and an error.
The functions discussed at #443 are also added
- `ByPublishDate`: Order contents by `PublishDate` front matter variable
- `GroupByPublishDate(format, order)`: Group contents by `PublishDate`
front matter variable formatted in string like `GroupByDate`
- `GroupByParam(key, order)`: Group contents by `Param` front matter
variable specified by `key` argument
- `GroupByParamDate(key, format, order)`: Group contents by `Param`
front matter variable specified by `key` argument and formatted in
string like `GroupByDate`. It's effective against `time.Time` type
front matter variable
