Update peerDependencies

[shoonia]

Allow using cssnano@6.0.0.

When I try to use cssnano@6.0.0 in my package dependencies

  "dependencies": {
    ...
    "cssnano": "^6.0.0",
    ...
    "htmlnano": "^2.0.3",
    ...
  }

I see the following error in the console

npm ERR! code ERESOLVE
npm ERR! ERESOLVE could not resolve
npm ERR! 
npm ERR! While resolving: htmlnano@2.0.3
npm ERR! Found: cssnano@6.0.0
npm ERR! node_modules/cssnano
npm ERR!   cssnano@"^6.0.0" from the root project
npm ERR! 
npm ERR! Could not resolve dependency:
npm ERR! peerOptional cssnano@"^5.0.11" from htmlnano@2.0.3
npm ERR! node_modules/htmlnano
npm ERR!   htmlnano@"^2.0.3" from the root project
npm ERR! 
npm ERR! Conflicting peer dependency: cssnano@5.1.15
npm ERR! node_modules/cssnano
npm ERR!   peerOptional cssnano@"^5.0.11" from htmlnano@2.0.3
npm ERR!   node_modules/htmlnano
npm ERR!     htmlnano@"^2.0.3" from the root project
npm ERR! 
npm ERR! Fix the upstream dependency conflict, or retry
npm ERR! this command with --force or --legacy-peer-deps
npm ERR! to accept an incorrect (and potentially broken) dependency resolution.

[maltsev]

I released new version htmlnano@2.0.4