Document how to preserve a desirable set of default options

[aronisstav]

Is your feature request related to a problem? Please describe.
If/when default options change, the output of a previously tested invocation of Concuerror may also change. Users should have a way to prevent this.

Describe the solution you'd like
There are currently two ways to pass options: arguments and 'test module attributes'. Combined with the preamble section of the output file, either can be used to keep track of the options used in a particular invocation.