Abuse error_handler to get free metacalls in r3

This uses the very risky '$handle_undefined_function'/2 export from the
r3 and rebar_agent modules to allow meta-calls that can support plugins
and all other rebar3 extensions.

This is nasty but very tempting. Currently we only support:

- r3:do(Command)
- r3:do(Namespace, Command)

There is currently no way to pass arguments to the function such that we
can, for example, run cover analysis or tests on a subset of suites.

With the new abuse of '$handle_undefined_function'/2, we can detect the
unused commands (since they are not exported) and re-route them:

- r3:Command()
- r3:Command("--args=as a string")
- r3:Command(Namespace, "--args=as a string")

Of course, in doing so, we make it impossible to use the 'do' provider
(as in 'rebar3 do ct -c, cover') since the 'do' function is already
required for things to work.

Since the previous function had very strict guards, we can, without
conflict, add manual overrides that simulate the meta-calls fine.

Sample run:
https://gist.github.com/ferd/2c06d59c7083c146d25e4ee301de0073

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