shell.cmd() was originally implemented in #866, however was not yet
exposed. This command should be ready for people to try out, so this
exposes the command by default.
This is still not a full implementation of everything in the
https://shelljs.page.link/cmd-design design doc, however this completes
the initial phase and is likely good enough to replace most use cases of
synchronous shell.exec().
Fixes#495
65baedbf52