This is an initial effort to address
#177

With this change, klog uses github.com/posener/complete and
github.com/willabides/kongplete to generate completions for klog's own
commands.

There are two coordinated changes that make this feature work:

1. When the `COMP_LINE` environment variable is set, `klog` runs in a
   special command completion mode. It parses the value of `COMP_LINE`
   (the current command line), prints to stdout a list of possible
   completions based on what is on the command line so far, and exits
   early.
2. `klog completion` prints to stdout a code snippet that defines a
   completion specification in the current shell. When this code snippet
   is executed in a shell session, the shell will thereafter invoke the
   same `klog` executable (with the `COMP_LINE` environment variable
   set) to generate possible completions.

For example, in bash, completions can be enabled by running

    source <(log completion)

Once bash completions are enabled, pressing tab after typing "klog "

    klog [Tab]

will present the available klog commands.

In the current implementation, the generated completions are rudimentary
and lack ergonomics.

There is zero test coverage for this new feature. Because it relies on
interaction with the shell, this feature is not easy to test.