Command aliases

Users should be able to define command aliases. Something like this in `~/.prideconfig`:

```
alias.git.incoming = git remote update -p; git log ..@{u}
alias.svn.incoming = svn log -r BASE:HEAD
```

And then the user can do this:

```
pride do incoming
```

Or maybe:

```
pride incoming
```

It's a question how one can specify different commands for different repo types.
