Error when invoking without -ignoretagsregex

```
$ python main.py
Traceback (most recent call last):
  File "main.py", line 211, in <module>
    os.environ["IGNORE_TAGS_REGEX"] = args.ignoretagsregex
  File "/usr/lib64/python2.7/os.py", line 471, in __setitem__
    putenv(key, item)
TypeError: must be string, not None
```

Workaround:
```
$ python main.py -ignoretagsregex dummy
```
