added option "both"

This option, inspired by Capture::Tiny's "capture", returns (in list
context) both the STDOUT and STDERR streams, plus the exit code. In
scalar context it's a shorthand way of specifying that STDOUT and STDERR
should be merged into one scalar variable (previously only possible by
specifying the "stdout" and "stderr" options).
overlooked a "=for test cut" in 36c22a6cfb2bfdfe5
restored simpler Synopsis

The Cheat Sheet is probably a little too overwhelming; this restores the
simpler introduction removed in e8c1d87c1d6ea1debb778.
added git commits to changelog
clarified documentation about avoiding shell

Documentation shouldn't so explicitly state that we "never" go through
the shell, since on Windows it's not so simple, and it's really up to
IPC::Run3 to handle that.
moved previously disabled tests into author test

There was a set of tests in 12_capture.t that was commented out because
they tested IPC::Run3 and not IPC::Run3::Shell, and the former might
change its behavior and break the tests. These tests are now in their
own author test.
used output_is in a few more tests

This is just to double-check that when streams get captured, nothing is
sent to STDOUT or STDERR.
documented what happens in case of a signal

It was previously not documented that there is no equivalent to
"allow_exit" for signals; they always cause a warning (if enabled).
Also updated changelog with documentation updates.
added note about development version numbers

After the next release 0.52, version numbers will be increased to an odd
number just after a release to indicate that they are in development,
and increated to an even number just before a release to indicate a
stable release.
clarify b69258b6d69e8e / warnings on signals

The previous commit made it sound like there is *always* a warning in
case a command gets terminated by a signal, which isn't the case if
warnings aren't enabled.