Supply customizable stdio channels to argparse

In order to properly test the program we need to have a way to
intercept data printed to the stdio channels. There are different ways
to accomplish that task. While it is reasonably easy to just start the
program as a dedicated process doing so properly may be problematic from
inside a test because either the path to the binary has to be retrieved
or cargo -- the entity which knows the path -- be invoked. None of these
approaches is very appealing from a testing and code complexity point of
view: an additional fork means additional sources of errors and
flakiness, executing cargo has the potential to even cause rebuilds of
parts of the program, and while we are already testing against a slow I/O
device this additional code running is unlikely to go unnoticed in the
long-term.
Lastly, doing so also means that we leave Rust's type safety behind when
dealing with errors that could be nicely match'ed on when the test
invocation is just a function call.
To avoid all this complexity we instead strive for basically just
running the main function.
This patch marks a first step towards achieving this goal. It introduces
the infrastructure to supply custom Write objects to the argument
parsing functionality. Once more we piggy-back on the command execution
context and add objects representing stdout and stderr to it. We further
ensure that this context is passed to the argument parser invocations.

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