add support for TOML config

[rumpelsepp]

Depends on #9

An optional config file can be created in the following locations; the first one which is found will be used. It is searched in these locations in this particular order.

• $PWD/gallia.toml
• $current_git_root/gallia.toml
• ~/.config/gallia/gallia.toml ($XDG_CONFIG_HOME is recognized according to the XDG standard)
• /etc/xdg/gallia/gallia.toml

A path can be enforced by setting the environment variable GALLIA_CONFIG (the env variable avoids a chicken and egg problem with the ArgumentParser).

The config is used only for setting the defaults for the cli args. The cli flags have the highest priority, all config values have a corresponding flag. The argparse.Namespace object will stay as the source of truth for all configuration values. I suggest namespacing the configuration values.

A config template can be created like this:

$ gallia --template

[rumpelsepp]

@peckto It is done.

here you can see it in action: 735ff48

Show all defaults for all commands:

$ gallia --show-defaults

Print a template with all config settings, commented.

$ gallia --template

If you create a config file and set a value there, you should see the default value changing.

Example:

$ cat gallia.toml
[gallia]
[gallia.scan]
target = "foo"
$ gallia --show-defaults | grep "foo"