[Feature Request] Add a long-long style for simple substution of long text

[zeigerpuppy]

It would be great to have access to a long-long style which could be used for a simple substitution of the shortname. This is useful, for instance, in a contract document where the user may want to have a shortcut for writing longer names:

---
acronyms:
  keys:
    - shortname: client
      longname: International Widgets and Spares Inc
---

e.g.

---
acronyms:
  keys:
    - shortname: Rmd
      longname: RMarkdown
   style: long-long
---

## long-long

First use: \acr{Rmd}

Next uses: \acr{Rmd}

Output:

long-long

First use: RMarkdown document

Next uses: RMarkdown document