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

to prevent any data from being overwritten

fixes #395