Add --rbrequire (-q) option to opal cmdline tool

[hmdne]

The short rationale behind it is that for now it's impossible to
require opal libraries like opal-browser (close to impossible - it
needs Gem paths) or opal-erubi (impossible, because it needs to
modify the compiler context to support .erubi files).

The longer rationale is that opal-rspec depends on Opal::CLI. We
can't test opal-webassembly due to that.

[hmdne]

A small request: can this be backported into the Opal 1.0 series? This will help me improve the documentation on some gems I manage.

[hmdne]

I just found out it's not necessarily true about testing. Still I think it would be a good addition, so one could do something like:

opal -qopal-browser -ropal-browser -c my_script.rb

[hmdne]

There is an opal-build command which actually has an -r switch which does what this proposed -q switch does. I don't know if having two commands doing actually mostly the same thing is really beneficial, but so be it :D.