✨ Add long-form option parsing

So far we could only add single-letter (POSIX) options: `-e`, `-n` and
so forth. Since we will run out of alphabet at some point, this commit
introduces parsing of multi-letter long-form (gnu) options (plus any
modern application should really support it anyways):
`--echo`, `--noline`.

Additionally, we support supplying long-form options that supply a value
both in the spaced (`--hist-limit 0`) and the equals (`--hist-limit=0`)
forms. Short, long, spaced, equals can be mixed and matched between
freely.

Lastly, we retain the ability to concatenate short options as before
(`-ne` is valid, as is `-P0` for the respective options above).

This should cover all bases and does not complicate the code too much to
keep a coherent overview.

Changed several code samples in the documentation to make
use of short- or long-form options to point out
possibility.

CHANGELOG.md

@@ -14,6 +14,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   This replaces previous `-P` history flag toggle. Use number to set amount of recent
   entries to display, `-P3`. To completely hide history use `-P0`.
 - Add nerdfont emoji set download with `-D nerd`
+- Add parsing for long-form options (`--private` instead of `-p` and so on):
+  Options requiring a value can be given both in space-separated (`--hist-limit 2`)
+  and equals-separated (`--hist-limit=2`) versions. POSIX option concatenation still
+  works (`-ne` to echo without newline).
 
 ### Changed