Allow passing common as platform

[beatbrot]

In some cases, it makes sense to explicitly request tldr docs for the common platform.

When running tldr curl on Windows, for example, I only get an explanation that I should pick the common platform if I were interested in the unix tool curl:

Unfortunately, if I try using the suggested command, I get the following error:

> tldr curl -p common
error: "common" isn't a valid value for '--platform <PLATFORM>'
        [possible values: linux, macos, windows, sunos, osx, android]

For more information try --help

In my opinion, common should be an option here.

[RuiNtD]

Bump. The option to use common as the platform is also mentioned as a supported option within the tldr page itself and is supported by the official Python and Rust clients, with support in the Node client planned (tldr-pages/tldr-node-client#484).