Is your feature request related to a problem? Please describe.

I find more intuitive if the memory/swap values are displayed in MiB instead of GiB when the value is less than 1 GiB, for better precision.
For example, display 512 MiB instead of 0.50 GiB.

Coming from i3-status, which does this by default, I find this more appropiate

Why does polybar need this feature?

I think that when values are lower than 1GiB is better to see their MiB value: we have an higher precision and the same number of digits.

Describe the solution you'd like

In addition to the already existing label tokens:

;   %gb_used%
;   %gb_free%
;   %gb_total%
;   %mb_used%
;   %mb_free%
;   %mb_total%

add something like:

;   %mb_gb_used%
;   %mb_gb_free%
;   %mb_gb_total%

that show the memory value as described earlier.

I'd like the same options available also for swap.