Add pass support

The [UNIX passwordstore called `pass`](https://passwordstore.org) is a very handy unix tool that allows to have GPG encrypted passwords stored in a git repository and being used as a password safe.

CodiMD CLI could integrate with `pass` by checking for its existence and reading login (i.e. `pass show demo.codimd.org/myuser`) data from there.

In order to make it easy to integrate, we should provide a function that abstracts provided password. Means check if a password is supplied by CLi parameter, check if passwordstore is there and if so, check for a password. When non of the above provides a password, show a prompt.

Just my rough idea, feel free to extend/change/improve.