Add support for generic option-lists like `.ackrc`

[Alhadis]

Description

This pull-request adds support for generic option-lists that use shell syntax for their contents:

# Comment
--key=value
--verbose
-f

Currently, this is limited to .ackrc files, but I suspect analogous formats might be added in future. Hence why I've named this language Option List instead of merely ackrc or something.

About .ackrc files

.ackrc is a configuration file read by ack(1) that has ~5,683 indexed results on GitHub, with a healthy ratio of users and repositories.

The dotless variant ackrc appears to have adequate distribution, with ~430 search results and an ostensibly healthy user/repo ratio. Code search currently makes it cumbersome to count the exact number of users, so @lildude will want to review the actual usage of ackrc files on GitHub.

Colour rationale

The colour I chose is a muted shade of the Shell language's colour (specifically, its saturation and brightness levels were halved). Since these option-lists are a "shade" of shell syntax, I figured the derivation was appropriate.

Shell:	#89e051
Option List:	#476732

Checklist:

• [x] I am adding a new language
  • [x] The ~~extension~~ filename of the language is used in hundreds of repositories:
    Probably. I'll leave the census to @lildude. :wink: For now, a search for files named .ackrc tosses up ~5,683 results.
  • [x] I have included a real-world usage sample.
    • ackrc: Sourced from larrylv/dotfiles, an MIT-licensed repository.
    • .ackrc: Written by yours truly. It adds Linguist-related filetypes ack(1) is missing.
  • [ ] ~~I have included a syntax highlighting grammar~~
  • [ ] ~~I have updated the heuristics~~

[Alhadis]

@lildude Don't worry, I haven't forgotten about Mermaid. 😜 Just had to scratch an itch or two with some simpler formats first. 😁