mako: add outer-margin

[dezren39]

outer-margin is like margin but different. it was added in 2022 as a setting to mako but is unavailable in the module.

https://github.com/emersion/mako/pull/395/files

*outer-margin*=_directional_
	Set outer-margin of each edge to the size specified by _directional_. See
	*DIRECTIONAL VALUES* for more information.
	This margin applies to the outside of the whole notification list.

control over each edge is desired:
- A single value will apply to all four edges.
- Two values will set vertical and horizontal edges separately.
- Three will set top, horizontal, and bottom edges separately.
- Four will give each edge a separate value.
- Four will set top, right, bottom, and left edges separately.