Create Piper class, for piping from/to spawned processes
Implement input handling for Piper
Wait for stdout/stderr threads to end
Use Thread#join rather than #value
Use Piper for asciidoc filter
Move gem and colorize_syntax filter tests to Piper
Remove unused foo.txt and bar.txt files
Remove remaining uses of systemu
Merge branch 'bug/jruby-pipes' into release-3.6.x

Conflicts:
	Gemfile.lock