Add customization file

[aronisstav]

The summary of commit 4cdf5aa is 'Show available commands if none is specified'.

This means that git issue == git issue help.

I'd rather have git issue == git issue list.

Someone else might want something else. Possibly a different something else for different issue repositories. Maybe even different something elses for different clones of the same issue repository.

Git manages such things via a .gitconfig file. Should something similar be implemented for git-issue?

[dspinellis]

I'd go for git issue help following the POLA. However, I see that git branch lists branches. I prefer sensible defaults over extensive configuration, but if you want to implement such a configuration option, I'll pull it in. Try to think of a way to express the rule in a general way, e.g. as an alias. Also look at how Git configures such options.

[aronisstav]

I will not work on this for the foreseeable future, but might get back to it at some later point.

[dspinellis]

Could be used to specify the long listing format (#42)