Add --ignore-untracked flag

I added a flag to ignore untracked files when performing a codequality check. I have a number of untracked files in my project and they were causing unnecessary noise.

Example:

```sh
mkdir test && cd test
git init
echo -e "from collections import defaultdict\n\nd = defaultdict(int)" > main.py
echo "from collections import *" > err.py
codequality --scm git --ignore-untracked .

codequality --scm git
# err.py:1:: 'from collections import *' used; unable to detect undefined names
codequality --scm git --ignore-untracked
#<no output> because the files are untracked

# Now add all the files
git add -A
codequality --scm git
# err.py:1:: 'from collections import *' used; unable to detect undefined names
codequality --scm git --ignore-untracked
# err.py:1:: 'from collections import *' used; unable to detect undefined names
```