Add a nimble clean command to clean up build files.

[davidgarland]

Having a built-in nimble clean command that cleans up any test or binary files that have been built via for instance nimble build would be nice. Not quite as pressing of an issue now that nimcache in the local directory isn't a thing, but it would still be a huge improvement over having to write

task clean, "Cleans up files":
  exec "rm <project_binary_name_here>"

for every project.

[jabbalaci]

I just noticed that "nimble test" creates a binary file for every test file in the tests/ folder. It would be nice if nimble test could do a cleanup, for instance via an option: nimble test --clean.

Edit: nimble test creates binaries for tests/*.nim files. Collect tests/*.nim, cut off the ".nim" extension, and you get the binaries. Under Windows add ".exe". These can be deleted IMO.

[dom96]

Indeed, Nimble should clean up test binaries by default. Seems like an easy PR ;)

This means that this issue has two tasks now:

• nimble clean
• nimble test cleaning the binaries it creates automatically.