added rbenv-check. checks rbenv installed and configured correctly

[dyladan]

Specifically it checks that:

1. rbenv bin directory is in path
2. rbenv in bin directory is executable
3. rbenv shim directory is in path
4. which ruby returns the shim ruby and not system ruby

It outputs in convenient color coded format that draws attention to problems.

[mislav]

Looks useful. Compare to the "doctor" script I created for the wiki.

However, what sense does it make for this to be an rbenv subcommand if you can't execute it if rbenv isn't in your PATH? The instructions for the command should then say to execute it with full path, e.g. /path/to/rbenv/libexec/rbenv-check.

Also, colorization should be off is standard output is not to a tty.

Also, not sure if we'll pull this. I'll let @sstephenson weigh on this one

[dyladan]

I created this because of a problem that I had with paths that was causing my to use the system ruby and it took me a few minutes before I ran which ruby and figured it out.

[dyladan]

oops I didn't mean to close this.

[mislav]

Closing in favor of official rbenv-doctor script that's being developed in a separate repo.

Thanks for the initial suggestions! I've initially based the logic in my script on the checks proposed above.