Add missing list styles

[archiloque]

Intended to fix #77 , I have two questions

1. I've added the list styles I found in asciidoctor's CSS and not only .unstyled. I'm not sure of the status of three case not found in asciidoctor's documentation :

• ol.unnumbered
• ol.unstyled / ol.none
• ol.no-bullet

=> Should I keep them for compatibility ? Should they be in asciidoctor's documentation (or maybe they are here because people expect them but you don't want to encourage people to use them) ?

2. I've checked the checklist case and the default case is not really good:

The code at templates/ulist.html.slim mention /could use &#9745 (checked ballot) and &#9744 (ballot) w/o font instead and I think it would be a good day, what do you think ?

File used for testing :

= Test

== square

[square]
* one
* two
* three

== circle

[circle]
* one
* two
* three

== disk

[disk]
* one
* two
* three

== none

[none]
* one
* two
* three

== no-bullet

[no-bullet]
* one
* two
* three

== unstyled

[unstyled]
* one
* two
* three

== arabic

[arabic]
. one
. two
. three

== decimal

[decimal]
. one
. two
. three

== loweralpha

[loweralpha]
. one
. two
. three

== upperalpha

[upperalpha]
. one
. two
. three

== lowerroman

[lowerroman]
. one
. two
. three

== upperroman

[upperroman]
. one
. two
. three

== lowergreek

[lowergreek]
. one
. two
. three

== no-bullet

[no-bullet]
. one
. two
. three

== unnumbered

[unnumbered]
. one
. two
. three

== unstyled

[unstyled]
. one
. two
. three

== checklist

* [*] checked
* [x] also checked
* [ ] not checked
* normal list item

[ggrossetie]

I've added the list styles I found in asciidoctor's CSS and not only .unstyled. I'm not sure of the status of three case not found in asciidoctor's documentation :

Good question, for unordered list the documentation says:

The unordered list marker can be set using any of the following block styles:

• square
• circle
• disc
• none or no-bullet (indented, but no bullet)
• unstyled (no indentation or bullet) (HTML only)

https://docs.asciidoctor.org/asciidoc/latest/lists/unordered/#markers

For ordered lists:

For ordered lists, AsciiDoc supports the numeration styles such as lowergreek and decimal-leading-zero. The full list of numeration styles that can be applied to an ordered list are as follows:

Block style	CSS list-style-type
arabic	decimal
decimal [1]	decimal-leading-zero
loweralpha	lower-alpha
upperalpha	upper-alpha
lowerroman	lower-roman
upperroman	upper-roman
lowergreek [1]	lower-greek

https://docs.asciidoctor.org/asciidoc/latest/lists/ordered/#styles

To summarize:

• no-bullet is an alias for none (and since none is shorter, I think we should promote none instead of `no-bullet but both are supported).
• unstyled can be found in the documentation.
• unnumbered can be useful if you want to remove the numbering (on ordered lists) but you want to keep the indentation.

So I guess we should support all styles.

I've checked the checklist case and the default case is not really good:

Indeed, I think the main issue is that the checkbox does not scale with the font size. Since we are using a large font size (for legibility) it does not look great.

Please note that the built-in HTML 5 converter will use checkbox only when the interactive option is defined. Otherwise it will use unicode ✓ and ❏ or Font Awesome icons check-square-o and fa-square-o if :icons: font is defined.

Using interactive checkbox can be useful in a presentation so we should keep this option but by default we should use unicode.
I'm fine with ☑ and ☐ instead of ✓ and ❏.

Also it seems that it's possible to change the size of a checkbox using CSS : https://stackoverflow.com/a/24688640/3600979
So we might want to use this solution to make it look better when [%interactive] is set.

Hope it helps and thanks for looking into it 👍🏻