Declare debug as an optional peer dependency

[ylemkimon]

Related: #135.

npm (npm/cli#224) and Yarn supports peerDependenciesMeta field in the pakcage.json, which allows to mark a peer dependency as optional.

[coveralls]

Coverage remained the same at 100.0% when pulling 50e86f7 on ylemkimon:patch-1 into d320c04 on follow-redirects:master.

[RubenVerborgh]

Not compatible with #135

[ylemkimon]

@RubenVerborgh Actually, it's compatible with #135. peerDependenciesMeta is supported by recent versions of package managers: npm/cli#224, yarnpkg/yarn#6671, and pnpm/pnpm#1486.

AFAIK, it's also possible to set peerDependenciesMeta without declaring peerDependencies. This way, it'll be completely backward compatible and compatible with other package managers. I'll update the PR based on your feedback.

[RubenVerborgh]

“Recent”, but this library needs to work on old Node versions too. That’s the problem of #135.

[ylemkimon]

@RubenVerborgh I've edited PR the branch to include only peerDependenciesMeta. This way it can support all versions. It seems I can't reopen PR. Could you reopen the PR?