Make default issue-sorting method configurable

[sideshowbarker]

This adds the ghi.sort option for the case where you want a different default sort method than created—e.g., you usually want updated instead. This change eliminates the need in that case to manually specify the -S updated option every single time you run ghi list. If you set the value of ghi.sort to updated, you can instead just type ghi and it will sort by updated instead.

[drazisil]

Merged into my branch here https://github.com/drazisil/ghi/pull/5