[refactor] config and cleanup

[subfuzion]

Updated config loading precedence, modified config options, and performed minor cleanup.

$ make install-cli
$ amp
# note `--use-config` and `config-used` options

$ cp $HOME/.config/amp/amp.yaml .  # or create a new config
$ VERBOSE=true amp config ServerAddress "127.0.0.1:5000"
$ VERBOSE=true amp config ServerAddress
# output should indicate using using the project `amp.yaml`, not config in home directory;
# output should indicate ServerAddress is `127.0.0.1:5000`
$ cat ./amp.yaml  # output should also indicate ServerAddress is `127.0.0.1:5000`

$ cp ./amp.yaml ./amp2.yaml
$ amp --use-config ./amp2.yaml config ServerAddress localhost
$ amp --use-config ./amp2.yaml config ServerAddress
# verify ServerAddress == localhost
# cat `./amp2.yaml` and verify again

$ rm ./amp.yaml && rm ./amp2.yaml
$ amp config ServerAddress
# verify ServerAddress == `127.0.0.1:8080`
$ amp config ServerAddress localhost
$ amp config ServerAddress
# verify ServerAddress == localhost
# cat `$HOME/.config/amp/amp.aml` and verify again
$ amp config ServerAddress "127.0.0.1:8080"  # reset to original value

$ VERBOSE=true amp --use-config ./amp3.yaml config
# verify following output:
Warning: unable to load ./amp3.yaml, using default configuration

$ VERBOSE=true amp --use-config ./amp3.yaml config ServerAddress foobar
# verify following output:
foobar
$ cat ./amp3.yaml
# verify the file was created and ServerAddress is set to foobar
$ VERBOSE=true amp --use-config ./amp3.yaml config ServerAddress
# verify also displays foobar
$ rm ./amp3.yaml

[bquenin]

LGTM

[freignat91]

LGTM
tested ok, fixe a little typo in test description (.aml -> .yaml)