File name too long

When I ran it with an example mitmproxy capture, I get the following error:

```
Traceback (most recent call last):
  File "/usr/local/bin/mitmstatic", line 11, in <module>
    sys.exit(main())
  File "/usr/local/lib/python3.6/site-packages/mitmstatic/__init__.py", line 58, in main
    with open(localpath, 'w+b') as f:
OSError: [Errno 63] File name too long: 's://[some long url here]'
```