Add subject prefix to b4-config

[emersion]

.b4-config has send-series-to to specify the mailing list to send patches to. Some projects also require a different subject prefix, because the ML is shared between multiple repositories. There is b4 prep --set-prefix but no way to set it in the configuration file.

[emersion]

It seems send-prefixes is what I want here. However, it's undocumented.

[ItsNayabSD]

@emersion ,

I believe it is --set-prefixes. Got from the b4 prep --help output. Something like the following:

b4 prep --set-prefixes "RFC WIP"

[emersion]

This issue is about the configuration file entry, not the CLI flag.