Add tags backend

[Timmmm]

This backend allows extracting the text of tagged text in the document into a JSON file. It allows specifying a prefix so you don't get the content of all IDs, just the specific ones you want.

For example given

Add is an instruction. [#tag_add_wraps]#Add wraps on overflow.#

When running

asciidoctor -b tags --require=./docs-resources/converters/tags.rb input.adoc

You would generate a JSON file containing

{
    "tags": {
    	"tag_add_wraps": "Add wraps on overflow."
    }
}

[Timmmm]

Corresponding PR in riscv-isa-manual: riscv/riscv-isa-manual#1769