Edit backup flags options based on a hook

[JoshLoecker]

Is your feature request related to a problem? Please describe.
I do not have a particular problem, but I have an enhancement I would like. I would like to allow an unlimited upload rate (--limit-upload) when on specific networks (e.g., local network), but limit it on others (e.g., non-local network).

Describe the solution you'd like
An option to export/append backup flags based on a hook would be ideal although I'm not sure exactly how to extract arguments from a hook and append them to the backup command