Provide a way to launch reporters in foreground

[claremacrae]

SystemLauncher currently runs the command in the background, I.e. does not block the running tests.

This has the benefit of allowing multiple test failures to be reviewed at a time, and for some diff tools, allows them to only appear to be launched once, and to show multiple differences in one window.

However, there are some cases where it would be good to run the launched process in the foreground.

For example

• Supporting SublimeDiff - see #43
• Implementing a reporter that launches diff or similar, to show differences on the command line output. This would improve the output of test failures in our implementation of CI builds, I think.

For example implementations, see

• Code in #43
• https://github.com/claremacrae/ApprovalTests.cpp.Nursery/blob/master/ApprovalTestsNursery/ForegroundSystemLauncher.h

And for uses of these, see the reporters in https://github.com/claremacrae/ApprovalTests.cpp.Nursery/tree/master/ApprovalTestsNursery/reporters

[claremacrae]

This was released in version 8.4.0