smarter shell code reloading, add env options for refresh_paths & refresh_paths_blacklist

Warn but succeed when atoms are passed to erl_first_files

otp21 eperm fix for local install/upgrade, no need to set uid/gid

otp-21 eperm fix: bump of erlware_commons and relx

- Introduce support of add and del operations for overrides
 - OTP-21 compatibility
 - Bump cth_readable for OTP-21 compat
 - upgrade relx to 3.25.0
 - upgrade bbmustache to 1.5.0
 - run compile provider in default namespace from bare compiling (fixes
   hooks for mix builds)
 - Resolve string vs list crashing rebar3 in erl_first_files
 - Create ERLANG_LIB_*_erl_interface environment variables only if erl_interface exists
 - Add hooks to the upgrade command
 - Add --start-clean flag to rebar3 shell
 - Auto-boot main app in OTP app project templates
 - Use maps for child spec examples in templates
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Merge tag '3.6.0' into refresh_paths

Bump to 3.6.0

- Introduce support of add and del operations for overrides
- OTP-21 compatibility
- Bump cth_readable for OTP-21 compat
- upgrade relx to 3.25.0
- upgrade bbmustache to 1.5.0
- run compile provider in default namespace from bare compiling (fixes
  hooks for mix builds)
- Resolve string vs list crashing rebar3 in erl_first_files
- Create ERLANG_LIB_*_erl_interface environment variables only if erl_interface exists
- Add hooks to the upgrade command
- Add --start-clean flag to rebar3 shell
- Auto-boot main app in OTP app project templates
- Use maps for child spec examples in templates

Add OTP-21.0 to Travis

Bump cth_readable for OTP-21 compat

Revert "upgrade certifi to 2.3.1"

upgrade relx to 3.25.0

upgrade certifi to 2.3.1

upgrade bbmustache to 1.5.0

Finish adding specs for private funs in rebar3.erl

- added type specs for following private functions:
    - run/1
    - set_options/2
    - test_state/1
    - safe_define_test_macro/1
    - test_defined/1

run compile provider in default namespace from bare

Resolve string vs list crashing rebar3

- resolves #1645

Create ERLANG_LIB_*_erl_interface environment variables only if erl_interface exists

Introduce support of add and del operations under erl_opts directive

Add hooks to the upgrade command

Add --start-clean flag to rebar3 shell

Allows override of apps specified in other on-disk options. Equivalent
to `--apps=""`

Fixes #1785

As requested in #1733

Logger OTP-21-rc2 support

Also small output fix in rebar3 shell

Fix override_deps test from suite rebar_compile_SUITE

Auto-boot main app in OTP app project templates

Use maps for child spec example

People are starting to expect these niceties! It's also easier to drop
the config when you know it's there than add it when you don't know it
exists.