use assert_root to check whether user is root when '-r' option is used

[bpostalci]

Closes #46
Old version:

$ undervolt -r
modprobe: ERROR: could not insert 'msr': Operation not permitted
Traceback (most recent call last):
  File "./undervolt.py", line 470, in <module>
    main()
  File "./undervolt.py", line 396, in main
    subprocess.check_call(['modprobe', 'msr'])
  File "/usr/lib/python3.8/subprocess.py", line 364, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['modprobe', 'msr']' returned non-zero exit status 1.

Fixed version:

$ undervolt -r
You need to have root privileges to run this script with these options.
Rerun with 'sudo'.

[A1exKH]

LGTM.