This change adds two new (hidden) CLI commands:
* `gen-bash-completion`: This command generates a bash completion
script for the CLI, storing it in the file specified by the 1st arg.
This fixespulumi/pulumi#1172.
* `gen-markdown`: This command generates a directory of Markdown files,
one per command, documenting the CLI commands and their usage.
I originally did these as separate scripts that we can use in our
build processes, but it was actually even easier to make `pulumi` able
to generate them for itself. The nice part about this is that we don't
even need to bundle additional assets in order to distribute e.g. the
bash completion scripts, we can simply tell people to run
$ pulumi gen-bash-completion /etc/bash_completion.d/pulumi
This can also be used in our upcoming Brew installer.
