reverse relx config so profile settings are used properly

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

Add excluded_apps rebar.config keyword

Listed applications are simply ignored.

Allows for configuration time sub-application building.

move handling of undefined app state to function

- 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.

fall back to .app.src file if .app file fails to parse

print and format error message for bad .app files and all bad configs

Show stacktrace to errors caught in rebar3 module.

only run clean hooks once

run hooks when building plugins

don't lose global plugin providers

Properly setup namespace for dependency provider hooks

make deps task listed in help and update zsh/bash completions

add Windows build badge

Update deps if their checkout doesn't match lock entry

Lock forwarding is what happens when the file on disk are on a different
version from what is specified in the lock file. Files on disk should be
updated to respect what's in the lock file.

A negative test has been added so that lock files that are outdated vis.
the underlying git and packages but match files on disk do not get
forwarded. This job is left to the `upgrade' command.

Run hooks for single application only once.

don't add `{extra_src_dirs, ["test"]}` to `test` profile

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

Properly initialize command parsed args in rebar state

methods for retrieving `src_dirs` and `extra_src_dirs`

update readme building instructions to prefer source

purge loaded code when it conflicts with project apps in tests