New colors theme method and improved colors theme docs

[blake-mealey]

Hey, a couple of quick suggestions I'm happy to submit a PR for if you're interested:

1. Add a new colors theme method command which points to magenta

I like to use a distinct and consistent color when telling the user they can use another command to help with something (e.g. from my CLI `This action requires a logged in user. Run ${magenta(`${brand} login`)} to log in.`). Currently the default command prints `Type ${magenta(`${brand} --help`)} to view common commands.` so I think magenta is a good candidate for this.

2. Document all the theme methods in the docs

Currently the docs don't document the following theme methods:

• highlight
• line

I especially wanted to use the same consistent highlight color as the default commands, and had been using cyan until I was snooping through the source and discovered the highlight method. I think it would be useful to expose all of them.

[kathawala]

Just submitted a PR which does what I think you want (instead of command, I just named it highlight). Hope it helps :+1:

[blake-mealey]

@kathawala not quite what I was after. I can already access the highlight color from the print.colors object. I would like a new color called command which is an alias for magenta. I also don't want it to print the full line that color, so I'd still want it to be part of the colors object.