Support setting untracked files as dirty or not

By using an enviromental variable, @emrox added a feature to enable or disable considering untracked files as dirty or not. I added an alias and a function to set the environmental vars.
I think a better name for the alias would be `git_untracked_files_dirty`. It shows that it changes the setting for Git, and secondly, the other available Git functions (in the `plugins/available/git.plugin.bash` file) use the underscore as a separator.