Introduce CLI options, improve usage instruction

Remove `version` command, keep `-v` and `--v` options.
Remove `help` command, add `-h` and `--help` options instead.

Rework usage instructions, split them into two sections - commands and
options. Change the formatting to be more consistent with other slc
commands like `slc clusterctl`.
Use SLC_COMMAND instead of CMD for $0
Handle missing `name` argument

Report an error when the required argument `name` is not provided
for one of the commands `add`, `remove` and `use`.
Merge pull request #3 from strongloop/feature/polish-user-experience

Polish user experience