* Implement partial include of source files.
The macro `{{include some_file}}` accepts now optional line number
arguments, s.t. the specified line range is included. The following
forms are supported:
* `{{include some_file::}}` is equivalent to `{{include some_file}}`
* `{{include some_file:from:}}` includes lines [from, infinity)
* `{{include some_file::to}}` includes lines [0, to]
* `{{include some_file:from:to}}` includes lines [from, to]
* Remove the special case IncludeFull which is IncludeFrom(0).
* Use Range, RangeFrom, RangeTo and RangeFull to represent include-ranges.
Also:
* Move out introduced methods as free functions.
* Introduce RangeArgument trait as long it is unstable in stdlib.
* Use itertools for joining of lines on the fly.
* Split tests.
* Simplify include file argument parsing.
* Make utils::string private and link collections_range feature issue.
