Improve documentation of stdout/stderr capture

[jimporter]

(Spun out from #33)

Currently, there's only one place that mettle's stdout/stderr capture is documented, and that's in the command-line option reference. This is way too obscure for most people to notice, so there should be some more detail in other places:

• Provide an example of output capture (and how to show output) in the Tutorial
  • This should have a separate heading that shows up in the page's ToC to help people notice that output capture is a thing
• Add more details about what test runs look like in Running Tests
  • Include some of the stuff from the tutorial, and especially cover verbose output
  • We should also add some stuff about showing how long tests take, doing multiple runs, etc
  • Maybe even pull out the command line reference into its own file
• Maybe add something on the main index about how mettle captures output? Try to keep the index pretty brief though...