Global configuration file and builtin configuration

[toyboot4e]

Hi, git-cliff is cool stuff!

1. Is your feature request related to a problem? Please describe.
I'd like to not put cliff.toml in my repository when I use git-cliff just as an alternative to git log.

2. Describe the solution you'd like
git-cliff locates cliff.toml in ~/.config/git-cliff/cliff.toml. If no configuraiton file is found, git-cliff uses default (builtin) configuration.

3. Describe alternatives you've considered
git-cliff looks into GIT_CLIF_CONFIG environment variable.

4. Additional context
Default configuration removes the need of copying cliff.toml, so it would help first-time users.

[orhun]

With the v0.2.* release:

• default configuration file is embedded into the binary and it is used if no configuration file is specified
• this embedded configuration file can be saved to cliff.toml via git cliff --init
• configuration file is read from $HOME/.config/git-cliff/cliff.toml if the path exists

Closing, thank you for submitting this issue!