BSDs are supported, but cannot be passed to the `--platform` option

[acuteenvy]

The client supports displaying BSD pages without specifying --platform (e.g. tldr pkgin), but displays an error when the option is used (e.g. tldr --platform netbsd pkgin):

$ tldr pkgin
no errors, the page is rendered as it should

$ tldr --platform netbsd pkgin
usage: tldr command [options]
tldr: error: argument -p/--platform: invalid choice: 'netbsd' (choose from 'linux', 'osx', 'sunos', 'windows', 'common')

[owenvoke]

This was probably implemented before netbsd was added to tldr pages. 🤔 I think it should be fine to add netbsd as an option. 👍🏻

I've created support for this here: #236