Add ability to customise number formatting when using `format` strings

[ammgws]

I was also wondering if decimal control could be integrated into the format string.

When thinking about things like the net or the speedtest block, it would be neat to be able to define the precision separately for the different displayed values. Ideally the syntax would allow to define all 3 arguments of the format_speed() function added in #704 / #707, like for example

Free: {free:3:G:B} for Free: 21.9GB

or

Uplink: {up:2:M:b}/s for Uplink: 50Mb/s

Originally posted by @GladOSkar in #714 (comment)

[GladOSkar]

I would like to work on this but really don't have the time currently, so if somebody else wants to i don't mind

[ammgws]

Covered by #1108