Allow emojis to be force enabled with --emoji.

[elyobo]

The previous behaviour had a default of enabling emojis for Macs, and
allowed explicitly disabling them.

This PR keeps that default but allows --emoji to enable for other
platforms if the user wants to. --no-emoji still works as before, but
due to the way that commander processes "--no-" flags (forcing the
default to true and overwriting the default value given in the .option
definition) it was necessary to switch the flag definition from
--no-emoji to --emoji.

Summary

Allows users to enable emojis on non-darwin platforms.

Emojis are a nice feature but are explicitly disabled on non-darwin platforms. There appears to be some interest in having them on other plaforms (see #960) and this allows that by making --emoji work as you'd expect (--no-emoji and --emoji are already accepted, but only --no-emoji works).

Test plan

On linux

# Forcing emojis off
$ node ./lib/cli/index.js --no-emoji
yarn install v0.24.0-0
[1/4] Resolving packages...
success Already up-to-date.
Done in 0.65s.

# Default behaviour; off on non-darwin
$ node ./lib/cli/index.js 
yarn install v0.24.0-0
[1/4] Resolving packages...
success Already up-to-date.
Done in 0.68s.

# Forcing emojis on
$ node ./lib/cli/index.js --emoji
yarn install v0.24.0-0
[1/4] 🔍  Resolving packages...
success Already up-to-date.
✨  Done in 0.67s.

Note that there is a flow type error in this PR related to process.stdout.isTTY, however this PR didn't add that check; it seems like it's a flow error that it wasn't already failing (I'm not familiar with flow though, so happy to be corrected here).

Another tweak would be to shift the process.stdout.isTTY check into the default value for commander.emoji, which would allow users to force emoji output even if stdout isn't a TTY - I'm not sure if this is ever desirable, but it's not possible to override at the moment and the change is simple.

[elyobo]

Test failures appear to be unrelated to the changes in this PR (tests are failing in the master from which I branched this).

[elyobo]

Worked around the flow problems with isTTY, although it still checks it (possibly an error that it's not a flow error?).

[bestander]

I think this will make emoji off by default, we have them on by default on Mac.

[elyobo]

@bestander, this PR keeps the default behaviour as is - enabled for Mac, disabled for the rest.

commander.option(
  '--emoji',
  'enable emoji in output',
  process.platform === 'darwin',
);

The third option to commander.option sets the default value for the --emoji flag to true if the platform is darwin.

[bestander]

Sorry, did not notice :)

[bestander]

Thanks for the PR.
Emojis FTW!

[elyobo]

NP, thanks for the merge, now I will no longer have emoji envy :D 🎉