neofetch: Added support for --memory_unit to change memory output unit.
neofetch: Added support for --memory_unit to change memory output unit.

Examples:

kib  '1020928KiB / 7117824KiB'
mib  '1042MiB / 6951MiB'
gib: '0.98GiB / 6.79GiB'

Precision level of GiB output is /not/ configurable at this stage in
time. This may be added later.

Closes #1388

dylanaraps/fetch,dylanaraps/neofetch