While 'do' skips state updates between subcommands so that:

    rebar3 do a, b  == (rebar3 a && rebar3 b)

The final state of 'b' does not need to be discarded and might in fact
be useful to get when dealing with Rebar3 as an API.

This can be done without breaking the equality relation already
established.

The shell agent allows to run rebar3 commands and autoload compiled
modules when that is done.

Listed applications are simply ignored.

Allows for configuration time sub-application building.

- moved path addition, config loading and app boot to before the shell
  is available
- apps successfully booting are in an INFO message, failed to boot into
  an ERROR message
- A warning is printed when apps are booted informing to please use
  releases for actual deployment, and is omitted otherwise.
- Some minor refactorings otherwise.

adding it results in ct test suites being compiled before they're
properly copied so changes made to suites between runs are not
reflected if a run fails before test dirs are copied

note that now ALL `src_dirs` across included profiles are
compiled. previously only the last included profile's `src_dirs`
were used

avoid repeated loading and unloading of modules

like `src_dirs`, `extra_src_dirs` are directories to be copied to
the `_build` dir and compiled. unlike `src_dirs` they are not added
to the .app specification

- The rebar package index files have been moved off the default path and
  will require a new `rebar3 update`
- Caching of downloaded packages automatically takes place in a path
  relative to the CDN used
- The cache path is not shared with hex as we now write and modify data
  in there arbitrarily
- Basic tests plus the working set for more of them is included

rather than profile root

fixes #415