Page.GroupByParam function internally uses Page.GetParam to get a
parameter value for a key of a page group but now Page.GetParam returns
a lowercase character string every time. It has no need to using
lowercase character string as a group key value and it confuse a
function user.
This fixes it to keep and return an original parameter string as a group
key value.
Fix#1564
Following issues are fixed
1. Can't access fields and methods specified in GroupBy call
2. PagesGroup doesn't contain Pages. It's always empty.
3. When GroupBy is called with Section key, it doesn't work as expected
`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
