Support multiple scripts and script arguments. refs #76

This PR adds support for multiple scripts and script arguments to mitmdump.
- add code to handle multiple scripts
- add code to properly handle script arguments on both windows and unix systems, as discussed in #76
- update docs
- add tests for new functionality
- mitmdump integration

**not** included:
- mitmproxy GUI integration

@cortesi, could you take care of the latter? My primary dev environment is Windows, so it's a little bit though to work on that efficiently.