Rewrite FormattedComponentArgument to use markdown

[Earthcomputer]

Rewrite the FormattedComponentArgument to use the following subset of markdown:

• *italic*
• _italic_
• **bold**
• __underline__
• ~~strikethrough~~
• [link](href)
• \*escaped stuff\*

Also continues to allow more formatting in the old way:

red{hello blue{world}!}

Supports legacy formatting codes via the braces syntax:

c{hello 9{world}!}

Designed to help along #464

[xpple]

Looks good. Though as the argument type does not just support formatting - it also supports styling - I would rename the argument type to StyledComponentArgument.

I will test the workings of the argument type later.

comments addressed and tests added