Add support for user-specifiable name for cache directory

[herber]

Users should be able to choose the name of gittar's directory in ~/.

Instead of .gittar users can choose any name.

Example

gittar.fetch

gittar.fetch('lukeed/polka', { name: 'custom' }).then(console.log);
//=> ~/.custom/github/lukeed/polka/master.tar.gz

gittar.extract

gittar.extract('lukeed/polka#master', 'foo', { name: 'custom' });
// Searches in ~/.custom/ for the tarball
//=> contents: foo/polka-master/**

[herber]

Thanks for the suggestions!

I added this feature, because some users might worry about the .gittar folder, instead of .my-app. They probably don't know .gittar, and would just delete the folder.