Add `slc arc` command

A part of https://github.com/strongloop/strong-arc/issues/380

The command corresponds to `strong-arc` from `strong-arc`.

I am using `strong-arc@0.3.0` as currently published in npmjs.org. We should upgrade the dep specifier to `strong-arc@^1.0.0` after Arc's GA release.

I am not sure what is the cause of whitespace changes in `man/slc`. My Ronn version is v0.7.3, perhaps the file was generated by an older version before?

/to @sam-github please review
/cc @altsang @chandadharap