Move default config to installed file

[emersion]

• [ ] This doesn't work well during development: /etc/mako/config is used as defaults, the user-provided config/flags are loaded on top, but /etc/mako/config might not be installed in which case mako won't run at all.
• [ ] Write docs

[RaoulHC]

Would it make sense to add SYSCONFDIR "mako/config" as an option here and provide a default file that can go there, but leave the rest of the default setting as is? That way it will still work even if it isn't present (and it probably shouldn't assume that it will be).