update relx to 3.16.0 to include color intesity update

Less color

Changes to how hex or packages may work in the future will necessarily
bring changes to the format of lock files.

This commit adds an optional framing for future lock files of the form:

    {Version, LockList}.
    <Whatever consultable attributes>

This format is supported such as the LockList is the current lockfile
contents, and will never have more information than it currently does.

Attributes can be whatever and are currently undefined.

Rebar copies will be able to:
- Keep using the core locklist (which avoids breaking the last year or
  so of community libraries using rebar3)
- Warn when it runs an outdated copy in comparison to the lock file
- Automatically rewrite lock files in the format it supports
- Augment or parse files in a version-specific manner.

This changes the usage interface slightly, but is backwards *and*
forwards compatible.

Add secondary hook for rebar_prv_compile

The provider is used for debugging to help displaying current rebar's state.

Usage:
  rebar3 state

upgrade relx to 3.15.0

Add test case for relx overlay vars

Ignore unknown warning when dialyzer < 2.8

Makes use of several var types: integers,
strings, binaries, binary strings and tuples.

check top level config for minimum or blacklisted  otps at start

erlc_compile, for before/after compiling .erls to
.beams, but before .app.src to .app

add user-agent to http request headers

don't lose overrides in an app when installing plugins it uses

Fix bash completion regression (cf66dfd6ba) and make lopt strings more resilient

don't strip the project apps when running `ct` with just a root suite specified

this ensures the project apps are compiled to `lib/` instead of `extras/`

Add module directory to include path

Several projects use an include path relative
to the project's root.
file:compile will look in three places for the include
files:
The current working directory
The directory where the module is being compiled
The directories given by the include option

To prevent future mistakes like the broken string introduced in
cf66dfd6ba, put opening and closing " on separate lines in each
multi-line lopts string. This also allows for cleaner (smaller) diffs
when removing/adding a flag.

cf66dfd6ba introduced a syntax error in the modified lopts string.
Without the fix, you'll provoke a shell error when trying to complete
`rebar3 ct --<something>`.

add profile option to clean task

deduplicate default test set generated by `rebar3 eunit`

local install and local upgrade

add unstable install/upgrade instructions to readme

this ONLY attempts to deduplicate test sets that are generated by
rebar in the absence of any user specified tests

Rework README and CONTRIBUTING documents