Add theme set option

[yhatt]

This PR will add --theme-set option to use additional theme CSS files. It can use the theme CSS created by the user! 🎉

Let’s say there is a theme CSS suitable for Marpit framework like this (css/additional.css):

/* @theme additional */

@import 'default';

section {
  background: #f80;
  color: #fff;
}

h1 {
  text-align: center;
  color: currentColor;
}

And slide.md:

<!-- theme: additional -->

# Hello, marp-cli :smile:

`--theme-set` option can use additional CSS themes by user.

The additional theme can use in theme directive of Marp Markdown by setting --theme-set option.

marp --theme-set css/additional.css -- slide.md

# It can specify theme by --theme option too
marp --theme-set css/additional.css --theme additional slide.md

--theme-set option recieves array, so you can add multiple themes.

# Add multiple themes
marp --theme-set one.css two.css three.css -- slide.md

# Add folder
marp --theme-set ./themes/ -- slide.md

This is the first step of Marp's better custom CSS support. It's related to yhatt/marp#1 and yhatt/marp#5.

Remaining tasks (will implement in other PR)

• Pass theme CSS path in --theme option directly (It allows anonymous theme)
• Track change of additional theme CSS in watch mode (-w)

ToDo

• Add tests