All the info function names started with "get_", which I removed to make
it easier for the user to configure the program to its needs.
Additionally I renamed some functions (e.g. get_ram_usage) to better
names, making it easier to extend the program with ram usage / total
functions.

add support to show used disk for a given mountpoint

Changed get_datetime() a bit so it respects the user-configured locale (to use time formats with strings for days, months etc.)

Same commit as last time, except it merges.

add a short sleep to reduce load

The buffer being hardcoded to 19 (the size expected from the default time format),
strftime() would fail on any format returning a longer buffer.
Changed it from 19 to 64 to accomodate longer formats.

Former : MemTotal - MemAvailable
New : (MemTotal - MemFree) - (Buffers + Cached)

Fixed indentation
Removed trailig white spaces