allow `src_dirs` and `extra_src_dirs` at the root of the rebar config

improve the rebar3 shell

Helps with integration efforts, but unfortunately can't support the
'-sname' and '-name' options, only '--sname' and '--name'.

Config files from the command line would expect the wrong format:

    {app1, [...]}.
    {app2, [...]}.

Instead of the correct sys.config format:

    [{app1, [...]},
     {app2, [...]}]

Not supported yet: the recursive file references documented in
http://www.erlang.org/doc/man/config.html

fixing plugin install and paths

Precedence still goes to shell_apps.

Change the order from load-config -> start-apps to load-apps ->
load-config -> start-apps

The feature may still be modified in the future.

Fix upgrade of atom-only packages

they would always be left unfound otherwise.

This allows proper checking of new configurations, deps, or plugins, and
makes sure they are detected during an active shell session.

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.

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