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.