The goal is supporting the special `all` platform that results in pages
for all platforms being listed when calling `--list`. It's part of the
tldr client specification.
However, `All` should not be a variant of the `PlatformType` enum,
because `Current` isn't a `PlatformType` either. Thus, we accept the
string `all` but convert it into the current platform when parsing.
For consistency, the same is done when no platform is specified, by
introducing yet another possible value `current` which is used by
default. This way, we get rid of the `Option`.
To simplify handling of os / platform arguments, a conflict between
`--platform` and `--os` was introduced.
The Apple computer operating system was renamed from "OS X" to "macOS"
in 2016. We should accept "macos" instead of "osx" in all our CLI APIs,
and the docs should also mention this alias instead of "osx".
For backwards compatibility and for compatibility with the tldr client
specification, we should still accept "osx" though.
This makes it compliant with the client spec.
The `-p` flag was previously used for the pager, using the pager now
requires using the long flag (`--pager`). Alternatively the pager can be
configured through the config file.
This commit also renames OsType to PlatformType.
