[Open Assets] srb2dl - SRB2 Content Downloader CLI

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.

• Fixed issue on some downloads, like builds, with getting error message about connection issues with SRB2 Message Board, even though site or internet is working.

• Fixed issue of not displaying search results of resources,
• Added checks for connection with SRB2 Message Board,
• Corrected displaying double quotes and other special characters in resource page's titles and filenames,
• More improvements for filename matching when upgrading resources.

• Added feature of upgrading previously downloaded resources,

• Added other features related to managing entries of downloaded resources,
• Corrected colors in messages,
• Other fixes.

• Added SRB2 VR's source code and its compiled binaries for Windows and Linux,
• Changed SRB2 Kart VR's source code link to proper branch,
• Changed link to SRB2 Persona .exe file,
• Added SRB2 Persona AppImage,
• Made more colored text for some messages.

• Added SRB2 Persona and its source code, GoldenTails' SRB2 AppImages, Lonsfor's SRB2 Kart, SRB2 Kart Moe Mansion AppImages to Custom Builds.