#361 Make the `--override` option chainable

$ borgmatic create --override key.a=... --override key.b=...

$ borgmatic create --override key.a=... key.b=...

Hi, First of all thanks for making this great tool! I've used it for a bit of time now and I really enjoy its simplicity and the powerfulness of the configuration file. I've a little suggestion for the documentation: it took me a long, very long time a few hours ago to realize that the `--override` argument took a list of arguments to override, instead of having to specify multiple `--override` ones that don't merge with each other. This means that doing this: ```shell $ borgmatic create --override key.a=... --override key.b=... ``` Is invalid and will result in only the second override clause being considered, which can be hard to debug as the documentation does not indicate how to perform multiple overrides. So I would suggest to merge the `--override` arguments where multiple ones are provided to the `borgmatic` commands, as that's what most CLI do and is what most end users will implicitly think to. Otherwise, I suggest to at least make the syntax clear in the documentation: if multiple items must be overriden, the following must be used: ``` $ borgmatic create --override key.a=... key.b=... ``` Thanks in advance!