feature: added a default placeholder color variable for the Input com…

[bolivierjr]

…ponent and added a color parameter to the placeholder mixin.

references issue #503

You are now able to change the placeholder color if needed through a few ways:

• Change the $input-placeholder-color in _harmonium-component-settings.scss from zip file.
• Use the harmonium cli to init/build new settings and add $input-placeholder-color to _harmonium-settings.scss (easier way).
• Use the placeholder mixin, that I added a parameter to, in your apps scss file. Which you're able to change just individual Inputs or all if needed.
  e.g.

<Input className="rev-Input--Foo" placeholder="foo" />`

then in your apps scss file for specific components

.rev-Input--Foo {
    @include placeholder($color-darker-gray);
}

[revelry-integrations]

🎉 This PR is included in version 6.2.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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