Add set_select_lowest to header
Add implementation of ws_set_select_lowest()
Add interface def. for ws_set_select_greatest()
Implement ws_set_select_greatest()
Merge pull request #363 from matthiasbeyer/add-set_greatest_lowest

Implement ws_set_select_greatest/lowest()
Add string.commands
Add stubs to string.c
Add string files to CMakeLists.txt
Add ws_builtin_cmd_strcat implementation
Add ws_builtin_cmd_substr implementation
Add static function get_strings_from_union
Add implemenation of ws_builtin_cmd_strcmp