Default header with more identifying information

Add an additional, default header which includes some identifying information. This would be useful for tracking naive abuse of swaks. Make it very easy to remove/disable for people who know what they're doing.

Possible information to include:

- local machine ip
- local username
- date/time
- reconstructed command line (minus passwords, elided data, etc)

Add specific command line option to disable this header. Thoroughly document the header's existence and the ability to disable it. Include note about disabling it "permanently" by adding the option to disable it in bashrc.

No files, tests, or entry points are named. Start by locating Swaks' command-line option parsing, default header construction, and documentation; done means the identifying header is default, a specific option disables it, and the documentation covers permanent disabling through bashrc.

Written by the indexing model from the issue text.