Adds -p and -P options for list

This is a quick implementation of what has been proposed in #75. Not sure you want it - it involves a slight interface change, as currently the `-p` option of the `list` command exludes pull requests.

These changes would follow the widespread standard, where a downcased letter means something positive and its uppercase variation is its negation.

`-p` for `--pulls` to list only pull requests
`-P` for `--no-pulls` to exclude all pull requests

It's also a little refactoring for the `!v.nil?` check, which is not so easy on the eyes.

Closes #75