Provider namespaces

The following now works:

```
λ rebar3 → provider-namespaces* → ./rebar3 new
app (built-in): OTP Application
lib (built-in): OTP Library application (no processes)
plugin (built-in): Rebar3 plugin
release (built-in): OTP Release structure for executable programs

λ rebar3 → provider-namespaces* → ./rebar3 lfe new
===> Command do not found in namespace lfe

λ rebar3 → provider-namespaces* → ./rebar3 default new
app (built-in): OTP Application
lib (built-in): OTP Library application (no processes)
plugin (built-in): Rebar3 plugin
release (built-in): OTP Release structure for executable programs

λ rebar3 → provider-namespaces* → ./rebar3 default new, new
app (built-in): OTP Application
lib (built-in): OTP Library application (no processes)
plugin (built-in): Rebar3 plugin
release (built-in): OTP Release structure for executable programs
app (built-in): OTP Application
lib (built-in): OTP Library application (no processes)
plugin (built-in): Rebar3 plugin
release (built-in): OTP Release structure for executable programs

λ rebar3 → provider-namespaces* → ./rebar3 lfe new, new
===> Command do not found in namespace lfe
```

Would need a merge and version up on the providers repo.

Attempt to address #69