Introduce template_dir config option

[mintelm]

This introduces a new config option called template_dir. What this new config option does it adds the possibility to define an additional directory to include when using load_template(). I have the 'issue' that my neovim configuration setup is as follows: all my plugins are under $RTP/lua/plugins/.... This means that I can not have my templates right where I want them to be (i.e. $RTP/lua/plugins/overseer/template/abc. This patch would fix my issue and I can imagine that others could also benefit from specifying an additional template directory.

[stevearc]

What do you think about changing it to template_dirs as a list of strings and using it for the definition of the default path?

[mintelm]

I think that's a nicer approach yes, I changed it and also rebased to latest main.
I was unsure which format to use for the directories, i.e. foo/bar or foo.bar, since I am not typically developing lua. What do you prefer? I used the dot style right now (I think this is typically used for loading lua modules via require()).

[stevearc]

LGTM thanks for the PR!