Add default completion algorithms

[rwxrob]

While internal completion is working, it does disable the default completion people have come to expect from Bash. For some that might be preferable behavior to disable accidentally passing things form the immediate working directory without wanting to.

Either way, all the standard completion algorithms/contexts from Bash need to be added as independent functions in a sub-package of their own and the file-system one should be enabled by default.

We could add additional completion algorithms based on what people request, such as dates, or dtime notation, or months, or planets, whatever so that people can modularly add it as needed. In fact, we could have a separate Register for completion functions that can be composed into stuff using the same method as cmdbox.New() does with cmdtab.Register (but maybe just a sub-package with an interface that people can fulfill is good enough).