Support CORS headers

[MarhiievHE]

metarhia/impress#1687 (comment)

• tests and linter show no problems (npm t)
• code is properly formatted (npm run fmt)
• description of changes is added in CHANGELOG.md

[tshemsedinov]

• It would be better to add HEADERS in Server class to do it once before server port open.
• We should support old versions of impress and config files, so we can't expect that config.server.cors.origin is present, need a dynamic check and use it only if it exists.

[tshemsedinov]

Moving all HEADERS to lib/server.js is not ok, it's better to hold them in lib/http.js, just patch from server, but exporting/importing HEADERS also not a good idea, I'd prefer to export function addHeaders from lib/http.js.

[MarhiievHE]

OK, do I need to delete the last commit before pushing the new implementation? Or do I need to just make a new fixup for this commit: 2125df0?

[tshemsedinov]

@MarhiievHE it is not important, I'll squash all commits to single landing this PR

[tshemsedinov]

Landed in 9b96150