Only run when I'm at the desk

It is of no use to me to get notifications while I am away from the
keyboard, i.e. when the screensaver is active.
Allow to specify a custom remote

Whenever I have a forked repository at Github, I am more interested in
changes that happen in the upstream repository, rather than my own fork.
This allows me to specify which remote I want git-dude to monitor, like
so:

```bash
$ git config dude.remote upstream
```

With a Git remote configuration like the below, it will monitor the
upstream repo instead of my fork:

```bash
$ git remote -v
origin  git@github.com:holmboe/salt.git (fetch)
origin  git@github.com:holmboe/salt.git (push)
upstream        git://github.com/saltstack/salt.git (fetch)
upstream        git://github.com/saltstack/salt.git (push)
```