Add flags and options to support user-defined templates

Is your feature request related to a problem? Please describe.

I define templates by myself and want to init with same mechanism which git-cliff already support by only for built-in templates

Describe the solution you'd like

# add directory which stores user-defined templates, 
git cliff --templates-dir <PATH> (ENV: GIT_CLIFF_TEMPLATES_DIR=)

# print all available templates, both built-in and user-defined templates, which is needed for providing completion in emacs.
 git cliff --list-templates

Describe alternatives you've considered

no

Additional context

• allow user to define his own templates
• give a higher priority to overwrite built-in templates if they have same names

See #370