Capture stdout/stderr of formatters

[zimbatm]

In order to be able to run the formatters in parallel, the formatter run interface should capture the stdout of the command.

Right now, each formatter is executed one by one, and its output displayed on stdout.

In the future, we want to run all formatters concurrently, and display their output in order, once everybody has finished. This is only possible by temporarily capturing their output in memory.

[Rizary]

@zimbatm can you refer to commit id that solved this?

[zimbatm]

That was solved by your commit, when you changed to use std::process::Command and output()

[Rizary]

Oh I thought it gets refactored. Noted.

[zimbatm]

It did, but Formatter::fmt also returns an Output