Fixed the "settings.yaml" file path for linux

The version of my python2 was `2.7.8`
I typed `python2 MarkupViewer.py README.md` in Archlinux,but it show me this.

```bash
$ python2 MarkupViewer.py README.md
Traceback (most recent call last):
  File "MarkupViewer.py", line 527, in <module>
    class SetuptheReader:
  File "MarkupViewer.py", line 541, in SetuptheReader
    'textile' : 'textile'
  File "MarkupViewer.py", line 501, in get
    return cls().settings.get(key, default_value)
  File "MarkupViewer.py", line 494, in __init__
    self.user_source = os.path.join(os.getenv('APPDATA'), 'MarkupViewer/settings.yaml')
  File "/usr/lib/python2.7/posixpath.py", line 77, in join
    elif path == '' or path.endswith('/'):
AttributeError: 'NoneType' object has no attribute 'endswith'
```
The "APPDATA" was a Windows env.I hope It would‘nt read the env 'APPDATA' in Linux,so I changed the code.