Add new flag to pass arguments directly to AFL++

[louismerlin]

The --afl-flags flag (-a for short) takes as an input any flag you would like to give to AFL++ directly.

Rationale

Sometimes, users want to pass custom flags to AFL++, to enable experimental features for examples.

Example

cargo ziggy fuzz -G 43 -a-G42

This command will place the second -G (with value 42) later in the list of arguments to AFL++, which will result in 42 being used for the biggest input size.

Closes #81.

[vanhauser-thc]

this can only pass one parameter, correct?
So I cannot do either -a -arg1 -a -arg2 nor -a "-arg1 -arg2" right? As it is it is good enough, might be good enhancement at some later point though.
thanks!

[louismerlin]

Hmm good point