Update CommandImplementation to handle large stdout

[TylerDixon]

What?

Markup renderers which were using the CommandImplementation to render the markup were hanging whenever large files were provided. So this PR:

• Updates the CommandImplementation#execute method to handle large stdout and stderr streams from called subprocesses
• Add a test to ensure that large RST files don't cause the process to hang

Why?

Previously, the implementation using popen3 would hang in the case of large files. Taking a closer look I tried a few things to see what was going on:

1. In the rest2html program, instead of writing the results of a large RST file to stdout, I processed the file, wrote it to a file, and just wrote a random short string (asdf) to the stdout buffer. This of course lead to the wrong return result, but, no hang!
2. Returning rest2html back to it's original state, I tried to read the stdout buffer with stdout.readlines before the wait_thr value. This also lead to no hang.

Based on these 2, we can see that the python and ruby sides were causing one another to hang:

1. The ruby side was waiting for status to be reported with wait_thr.value.success? before reading the stdout buffer
2. The python side was hanging because the stdout buffer was full, and therefore wasn't exiting to return a status.

The fix

capture3 is a wrapper around popen3 which creates new threads to read in stdout and stderr while waiting for a status to be reported. This means that we don't end up hanging due to a full buffer. The capture3 implementation can be seen in:

https://github.com/ruby/ruby/blob/83f02d42e0a3c39661dc99c049ab9a70ff227d5b/lib/open3.rb#L648-L677

[nasquasha]

oop CI looking real mad

[kenyonj]

closing in favor of #1814