Add some mapgen-related mods:

* snow
* moreores

Signed-off-by: Xiong Changnian <xiong@cpan.org>
Add:

* moretrees

Signed-off-by: Xiong Changnian <xiong@cpan.org>
Merge branch 'feature-mapgen' into devel

+ snow
+ moreores
+ moretrees

Signed-off-by: Xiong Changnian <xiong@cpan.org>