Post new note while also setting the alias

As an extension to https://github.com/hackmdio/codimd/pull/673 it would be nice to be able to create a note including an alias. I'm not particularly picky about how that would be done. Two ways come to mind:

- keeping `/new` and adding a parameter `?alias=the-awesome-alias`
- using `the-awesome-alias/new` (or the other way 'round)

This would enable the CLI to determine the url, making imports easier. This was - not as precisely - mentioned in https://github.com/hackmdio/codimd-cli/issues/7 already.