Use IO.popen to access copy command.

[nuclearsandwich]

I was investigating the need to use echo -e to process
backslash-escaped characters on Linux and was having a ton of trouble
getting the `` command to put the emoji character through the pipe in a
way that it be interpreted correctly. I also ran into the "hanging"
issue described in bintest/emoji-cli.rb#14 which I tried to strace

...output abriged...
write(1, "Running command /usr/bin/echo -n"..., 51Running command /usr/bin/echo -ne '🎈' | xclip -i) = 51
write(1, "\n", 1)                       = 1
pipe2([3, 4], O_CLOEXEC)                = 0
clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7fd0005409d0) = 15261
close(4)                                = 0
fcntl(3, F_SETFD, 0)                    = 0
fstat(3, {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0
read(3, 0xacb940, 4096)                 = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=15261, si_uid=1000, si_status=0, si_utime=0, si_stime=0} ---
read(3, "",

When using xclip it looks like parent-readable end of the pipe (file descriptor 3) is not being closed which could mean that xclip is potentially still waiting for more data from the parent-writeable end of the pipe (file descriptor 4) which is weird because that was closed above.

I think this must have something to do with how backticks function in mruby: so near as I can tell the shell commands are processed with sh -c BACKTICK_CONTENTS but that should still result in a successfully piped emoji character unless sh, echo, or xclip isn't properly reading it as unicode data.

I still don't know exactly why the echo command wasn't working on Linux and I'm a bit too unfamiliar with mruby to really dig into it as I am able to run sh -c "echo -ne 🐧 | xclip -i" and have it copy the penguin emoji to my selection.

Since the mruby-io mrbgem was already present, I rewrote the copy method to use IO.popen rather than backticks to directly write the emoji character to the standard input of the copy command's process.

I haven't yet been able to test this for Mac OS but we can always make it a Linux specific change if it doesn't work or the the backtick of echo is preferred.

cc @muan for review and Mac OS testing.

We might also be able to revisit the tests now that the hang has been resolved.

[muan]

THANK YOU!
I'm not ignoring this PR! just can't get docker to run on my computer again :( Will try again at a later time.

[nuclearsandwich]

I'm not ignoring this PR! just can't get docker to run on my computer again :( Will try again at a later time.

Oh no! 🐳 best of luck!