[Open Assets] srb2dl - SRB2 Content Downloader CLI

Fix for "Argument list too long" that occured when trying to choose other addon to download (https://github.com/Bijman/srb2dl/issues/13).

Features:
- Added support to download Ring Racers resources,
- Included SRB2 Kart Neptune, Ring Racers, Ring Racers Noire.

Fixes:
- Fixed issue of not detecting, if resource is the same using new method,
- Minor fix for detection of SRB2 configuration directory.

Other:
- Updated README.md.

• Added Ports sub-categories from SRB2 MB,
• Fixed blank entry, when there is no listed addons.

• Added Multi-Category category from SRB2 MB.

Happy New Year. Here is a new update to srb2dl with below changes:

Features:
- Added SRB2 category - 3D model,
- Added addon's history selection when searching addons.

Fixes:
- Added additional header in curl command due to issues with showing search results for addons,
- Fixed issue of not finding srb2kart configuration directory when environment variable SRB2DLAUTODIR is enabled.

Other:
- Changed selection in addon's history to match to how it works in --history option,
- Updated year in LICENSE.

Features:
- Added option "--history" for selecting version of resource and downloading it from mb.srb2.org by addon ID (issue #3),
- Added option "--id" for downloading resources from mb.srb2.org by addon ID (issue #7).
- Added source code and binary links to SRB2 rphys, SRB2 TSoURDt3rd, SRB2Kart Galaxy, SRB2Kart HEP and SRB2Kart Saturn,
- Updated links to SRB2/SRB2Kart builds.
- Removed links to source code for SRB2 Uncapped Plus and SRB2Kart Moe Mansion.

Fixes:
- Fixed issue of aligning "Open Assets" label with addon, which caused (issue #9),
- Fixed issue with running curl if there are no files (issue #10),
- Fixed filtering by reusable/open assets,
- Fixed issue with adding to database in some cases (for example links to SRB2 archives from "https://files.srb2.org/srb2.org/history"),
- Fixed issue with empty search author results due to square brackets in search URL.

Other:
- Updated README.md

• Fixed displaying messages for BusyBox's sh,
• Fixed issue with not to be able to search addons,
• Aligned messages to look a little bit better.

• Downloading files is available for Linux systems (Rocky Linux or Ubuntu 18.04 LTS) with curl not having option "--output-dir",
• Added environment variable SRB2DLDEBUG for verbose output from script.
• Removed link to binaries for SRB2 Uncapped Plus, since site "http://srb2.mooo.com" is down.

• Added function to download addons to selected subdirectories within SRB2/SRB2Kart configuration directory. To activate it, you need to add export SRB2DLAUTODIR=1 in shell configuration file. Keep in mind that, parsing download path argument to script is disabled, when this variable is set.
  
• Updated link to SRB2 Persona build,
• Switched positions of options,
• Added download paths to script's database,
• Other fixes.

• Fixed issue of script not executing properly with custom path.