The custom sort functions used from the templates had some subtle data race- and related issues,
especially when used in the single page template.
This commit fixes this by making copies and protect the read and writes with a RWMutex.
The results are cached (it will typically be invoked *number of pages* times with exactly the same data).
This is, not surprisingly, also faster:
```
benchmark old ns/op new ns/op delta
BenchmarkSortByWeightAndReverse 14228 491 -96.55%
benchmark old allocs new allocs delta
BenchmarkSortByWeightAndReverse 1 0 -100.00%
benchmark old bytes new bytes delta
BenchmarkSortByWeightAndReverse 32 0 -100.00%
```
Fixes#1293
`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
