Color Generator for the cli

Would be awesome if you can change or create new colors via ionic cli.

Giving the cli the name and a hex code for a color and it change the variables.scss file with all the generated colors.

Like
```ionic colors primary #5ca3ff```

and then changes `variables.scss` to
```
...
/** primary **/
--ion-color-primary: #5ca3ff;
--ion-color-primary-rgb: 92,163,255;
--ion-color-primary-contrast: #000000;
--ion-color-primary-contrast-rgb: 0,0,0;
--ion-color-primary-shade: #518fe0;
--ion-color-primary-tint: #6cacff;
...
```

Locate the Ionic CLI command entry point and inspect how a project’s variables.scss file is found and edited. Implement the requested color command so a name and hex code generate the shown color variables in variables.scss, then verify the file contents for a sample color such as primary #5ca3ff.

索引モデルが issue の本文から書いたものです。