Fixed my_configs.vim's name and added OS-specific files

* Renamed my_config.vim to my_configs.vim to comply with .vimrc provided
* Added ubuntu.vim and windows.vim files in vimrcs as a place for future
OS-specific configurations. They will be edited in their branches.
* Added proper source commands to my_configs.vim to load mentioned OS-files
Merge branch 'master' into windows